Take his goal away and he was poor at the weekend.and he does tire, ask John Terry
But i also think its because the balance in Midfield has been wrong now, for a few seasons, which possibly leaves Petrov open the stick he gets (unfairly). In a more balanced midfield he will probably be able to conserve his energy levels throughout a game a little better. It was Martin O'Neil who made him a sitting midfield player, and if he had the energy to play there, then why did he not play there before?
Our midfield options have been that poor since, that he has been stuck there, which points to the good job he has done in that position.
Its no coincidence that Mcleish is getting him a little more forward now, and the added abilities of a (hopefully) hungry Jenas, will balance things out.