A lot of times I just have 15 minutes between 2 matches. Cup and competition, Competition and Champions League.
When the first match is ended, I can't change anything in my formation because there are too few minutes left before the next match will start.
Yesterday I wanted to play with a kind of B Squad in the CL because of the fact I was already qualified as number 1 in the group for the next stage. The 2nd match which was competition and I couldn't give change my starting formation during the first match so I started also with a B squad in the 2nd match.