Do you watch Real Madrid play with Ruben de la Red and Gago? It's a great partnership until the Spaniard's career ended. I think Gago and Vidal can have the same partnership.
If the club still had Diego...
Matri
Perotti ---- Diego ---- Krasic
Gago ---- Vidal
Well I'm sure that's the same as playing with:
Gago, De la Red; Ronaldo, Özil, Di Maria; Hiaguin.
Compare the line ups, maybe, just maybe, that has something to do with it. La liga is a lot different than Serie-A and demands other qualities from the teams. I doubt Gago-Red would have reached CL final. I'm not saying they are bad but that if you if those kind of players on the pitch you have to rely on a lot of possession since your defence will leak.
Regarding Diego; Diego doesn't know when to let go of the ball. I can't say how many times I ended up disappointed. Every attack when he got the ball you knew he'd run with it until he eventually would get caught. I'm happy he left and last season he was terrible. Considering what we got for him and what level he showed back in Germany this season we should consider ourself happy. The only reason I felt sorry for him was that he isn't really suited for the position he plays in, I consider him as a SS and should be drawn higher up in the pitch than he currently is and was. Though I strongly believe he himself consider himself as some form of trequartista.
Naturally I could be wrong, but this is my opinion.