The four duels are...:
Manchester City vs Real Madrid
Atlético Madrid vs FC Bayern München
Villarreal CF vs Liverpool FC
Shakhtar Donetsk vs Sevilla FC
Let's start with the four Champions League duels. For sure, an important question to be answered before appointing semifinal referees is "Who should referee the final(s)?". In my view, this is clear since last week. Swedish Jonas Eriksson (photo) will have the honor to take charge of the Champions League final 2016 by 99,9% for my taste. His only real competitive, German Felix Brych, did well in the Barca vs Atlético game - also UEFA has this impression apparently. However, most decisions went against Atlético that made them raise strange conspiracy theories after the game. If Bayern qualify, Brych is out as he is from Munich, Germany. If Atlético qualify for the final, he is very likely out as well, as his appointment would create fuss in the media even before the very first whistle is blown. So... bad luck for Brych, he will have to wait another season. With Eriksson, UEFA exactly knows what they get and despite a crucial mistake by his first assistant referee in the Bayern vs Juventus Round of 16 match, he did quite well. So, my premise for the following is: Eriksson gets the UCL final. Another premise is: Nicola Rizzoli will get no further match before EURO 2016. Those who have seen Atlético vs Barca last week know why.
Brych should be rewarded for his good performance in Barcelona. As he did not handle Manchester City or Real Madrid in this season's K.O. stage, it is quite probable that UEFA will appoint him for one of this tie's legs. Usually they appoint a referee, who comes from the same nation like a team that joins the other semifinal, for the first leg to maximize the perceived level of impartiality. Therefore I expect Felix Brych in Manchester City vs Real Madrid.
Another interesting question is which English referee will be appointed for one of the Atlético vs Bayern legs. Definitely there will be only one English referee team in the semifinals (the only other option would be that one of Martin Atkinson / Mark Clattenburg gets the final...) as the 1st and 2nd leg referee teams should normally come from different national associations. While Clattenburg had a decent outing in the Atlético vs PSV Eindhoven Round of 16 match - and as Atlético won, there won't be that much prejudices; appointing Clattenburg thus would be accepted -, Atkinson did not perform well in Juventus vs Bayern. Personally I think that Atkinson got enough last season and is not at the peak of his abilities at the moment. Therefore I would prefer Clattenburg for a semifinal. It might be that UEFA sees it vice versa though. Analogously to Brych, I think UEFA will appoint an English referee for the match in Vicente Calderón. My prediction thus is: Mark Clattenburg to referee Atlético Madrid vs Bayern München.
Now it becomes more difficult. Someone who cannot be ignored after his widely good performances this season and last week is Turkish Cüneyt Çakır. I think the "closer" duel of those 2 UCL semifinals is the German-Spanish one. For this reason - and as Bayern's stadium is usually a difficult turf for UEFA officials - I think Çakır will take charge of the 2nd leg of Bayern München vs Atlético Madrid.
The biggest doubt I have is about who should referee the 2nd leg of Real Madrid vs ManCity in Santiago Bernabeu. Maybe this match is partly decided after the 1st leg. Maybe not. It has to be someone with a decent level of experience in big games for sure. Normally the most logical conclusion would be Björn Kuipers. His crucial mistake in Benfica vs Bayern last week was however a number too blatant and clear. I am not sure whether a clear 7.9 in a quarterfinal rules you out of a semifinal - but I think so. If we ignore him for a moment, there are many alternatives.
1) Damir Skomina. He has had a pretty good season so far and did some big games, mostly with a good performance and invisible appearance (low number of cards expect last week's Spanish UEL derby). I have not watched his performance in Sevilla last week, but it seems to have been quite good from what I heard. If so, then a UCL semifinal would make sense.
2) Szymon Marciniak: A rising star who is collecting experience in really big games this season. He did well in most of his matches, e.g. Real Madrid vs AS Roma or Bayern vs Benfica. A UCL semifinal would be deserved, but of course quite early in his career and a bit too risky. From a psychological point of view, he should be appointed for a UEL semifinal, but not for UCL.
3) Viktor Kassai & Milorad Mažić: Both did not always show the best performances in the early phase of this season and both somehow had to recover after WC 2014 (Kassai as he was not nominated for it, Mažić as he did not play a good role in Brazil). Kassai seems to have performed well in St Petersburg and Madrid - Mažić did overall well in ManU vs Liverpool. The Serbian however struggled a bit in Paris vs ManCity, there were 2 difficult penalty area decisions to take and, at least in my view, one of them was wrongly taken. Anyway: under the premise that Atlético vs Bayern goes to Çakır and Clattenburg, there is no room in UCL for both referees, as they handled either ManCity or Real Madrid in the quarterfinals.
4) Pavel Královec or Svein Moen: Both should not remain unmentioned. Královec had a good season and was my favourite for the UEL final for a long time. But his last performances were below expected level due to too many crucial mistakes. Svein Moen delivered solid, but never completely good performances this season from my point of view. Gent vs Wolfsburg showed that he has one huge problem: a lack of acceptance (as a result of missing tactical cleverness, I would dare to say). I think both should not get a further match before EURO 2016.
So, to come back to Kuipers, I don't think that he should be ruled out too early. But I would prefer Skomina, who also has semifinal experience and would deserve it more or less. No doubt that Björn Kuipers in Real Madrid vs ManCity would be a good choice as well (as you can argue that his wrongly interpreted SPA/DOGSO situation was 1 mistake and no sign of a systematically problematic performance).
In conclusion, I would choose:
Manchester City vs Real Madrid: Felix Brych (Germany)
Atlético Madrid vs Bayern München: Mark Clattenburg (England)
Bayern München vs Atlético Madrid: Cüneyt Çakır (Turkey)
Real Madrid vs Manchester City: Damir Skomina (Slovenia)
and Jonas Eriksson for the UCL final.
For Europa League, there are some other names to add: Above all, Scottish William Collum who did not have his best season, but did well and on the expected level in Bayer Leverkusen vs Villarreal CF in UEL Round of 16. A semifinal would be the logical next step. Shakhtar Donetsk vs Sevilla FC - without any doubt a big game - would be an opportunity. Then, there is Italy's Gianluca Rocchi, who also did well this season and could be rewarded with a semifinal or even the final. As there is no other Italian (assumed Orsato won't get a match and Rizzoli is out after his performance in Madrid), it would be no surprise to see Rocchi at Anfield Road (he may not handle Shakhtar as an Italian).
As previously outlined, I think Kassai and Marciniak should get futher matches, but not in UCL. In case of Mažić, all depends on how UEFA has seen the penalty area decisions in Paris. If he got a good mark, he would be a candidate for the final in Basel.
In conclusion, I would choose:
Villarreal CF vs Liverpool FC: Viktor Kassai (Hungary)
Shakhtar Donetsk vs Sevilla FC: William Collum (Scotland) (more conservative alternatives: Martin Atkinson or Björn Kuipers)
Sevilla FC vs Shakhtar Donetsk: Szymon Marciniak (Poland)
Liverpool FC vs Villarreal CF: Gianluca Rocchi (Italy)
and either Milorad Mažić, Szymon Marciniak or Damir Skomina - depending on the semifinal appointments - for the UEL final.
However, all these ideas are like a lottery and surely depend on several things most of us cannot and do not know. What are your thoughts?
Referee Observers for next week:
Manchester City vs Real Madrid: William Young (Scotland)
Atlético Madrid vs Bayern München: Alfredo Trentalange (Italy, FIFA ref. committee)
Villarreal CF vs Liverpool FC: Murat Ilgaz (Turkey)
Shakhtar Donetsk vs Sevilla FC: Stefan Ormandjiev (Bulgaria)