From the players available it seemed as though there were two options as to who could play on the left v PSG - KDB or Silva, with the other player taking up the free role behind Aguero. As KDB is far more of a goal threat than Silva it makes sense to play him in the more advanced position and last night, once again De Bruyne came through with a vital goal.
The problem is that Silva doesn't have the energy required to play wide left. He can't get up and down the wing in the same way that Navas does on the right. It is the same as playing Yaya in a midfield two, it just doesn't work. Silva's best performances for us have come when he is played behind the striker, or as the right sided player in an attacking 3. When played on the right he is able to cut inside and he knows how to utilise overlapping runs from the fullback - his link ups with Zabaleta the second time we won the Premier League were a key part of our attacking play.
I agree that if we play Silva on the left in the Semi Final it is going to cause us a lot of problems. The rest of the team is well balanced - Navas may lack quality in his delivery but he's extremely hard working and brings pace to the team, especially on the counter. Fernando and Fernandinho have been excellent in the centre for us in Europe this season... KDB is having a fantastic season and then there's Aguero up front.
It will be very interesting to see what formation Pellegrini goes for in the Semi Final. Hopefully the whole squad will be fit and he will have options, but the left hand side is a concern. A better team than PSG would have exploited that weakness.
Silva is undoubtedly one of our best players, but similarly to Yaya I think it is a case of either play him in his best position or don't play him at all. If Pellegrini sticks with this formation for the Semi Final I'm just not sure who else we could play wide left... but De Bruyne should retain the advanced role, he's earned it.