It was surely going to be the case, after forming such a strong partnership with Gareth Barry in midfield for the past 2 seasons, that Petrov would take a few games to get used to playing with a new CM in whatever system we played this season. The same goes for the rest of the team.
This appears to have been overlooked by many and MON villified for playing him as part of CM, yet against both Man City & Chelsea's quality midfields I thought he was simply outstanding as others have said.
I do agree against teams that play a more physical game like Wigan, Blackburn et al MON may need to fall on the 4-5-1 but in the last 2 home games the 4-4-2 with Petrov & Sidwell has worked a treat.