You have the same problem as Pellegrini, you want a direct replacement from our squad for the injured first teamer. Obviously that won't work; you won't get players of that quality willing to be back-ups and if they've not been playing regularly they're not likely to be able to just drop into place and there be no difference.
What we need to do is adapt to our situation. With the players we had available we should have been able to beat an injury ravaged Liverpool at home and at least had a good go at getting a draw in Turin against a struggling Juventus. As it was, Pellegrini set up exactly the same way he would have done had we had a fully fit squad, just using different players and we got mullered in both games. Liverpool got 4 and probably should have had 6/7, whilst Juve only managed one but in truth created 5/6 really clear cut chances.
With Klopp's arrival and their lack of fit strikers, Liverpool were always going to flood the midfield and implement a high press. What did Pellegrini do? He played our two least mobile holding midfield players, leaving Delph and Dinho on the bench and played Mangala, our least confident centre half in possession of the ball. Suicide.
Against Juve 0-0 was a brilliant result for us. So why not sit deep, pack the midfield and play with Navas, Sterling and Aguero on the break? We'd have had a good chance of getting a 0-0 or even knicking it. But no, we play one way and one way only. We try to dominate possession, with our centre halves on the half way line and get exposed over and over again by a team with a tactically astute manager. If Dybala and co. could finish it would have been every bit as embarrassing as the Liverpool game.
This "We will not adapt to the opposition, they will worry about us!" attitude is great if you're the best team in the world and have Messi, Iniesta and Neymar, but we're not! We're lightyears away from them at the moment and should be preparing for each game individually.