I agree that Mahrez probably won't start but to boldly suggest he won't be 100% committed in a Champions League semi-final is crazy when you consider he's part of a very, very select club of players in Manchester City's history.
In fact, not only is he one of only five players in City's history to score in a game of such magnitude, he's also one of only two players (along with De Bruyne) to score more than once in a Champions League semi (4 goals).
He's a big-game player and if he starts he will produce, as he often does. Even on an apparently "off day" the other day against Leeds he still racked up two virtually identical assists for Gundogan. He always produces.
Since he's arrived, in each season so far he's contributed 24 G/A, 29 G/A, 23 G/A, 33 G/A, and 27 G/A so far this season. People call him "Ffs" when he's one of our most consistently productive and deadly forwards.
I agree that Pep will probably opt for Gundogan and Bernardo together so that De Bruyne has space and time provided to him but if Mahrez does start he'll do what's required of him for a game of this level, as he has done before.