I agree about the new manager bounce myth, but in this case we have a side that (in most people's view) are underachieving somewhere between slightly and severely.
For all the defence being crap, the mere presence of Terry will give them an extra 20%. For the attack - we have the potential... I don't think we need a big shift to get an immediate effect. Plus there are a lot of players with entry to prove to the new boss.
It may be a while till we the the full effect, but I can easily see an immediate impact from the new team.