The Serbia international believes the problems have been caused by the players failing to replicate the attitude of last term.
He said: "We started well against Juventus but after that it was too easy for them. We can improve 50 per cent more on this performance.
"We are playing differently this year with more emphasis on attacking but everyone also has to be focused on defending. This is what helped us last season.
"It's all about our concentration and motivation. We have to think that every situation is dangerous and cover well. It's more of a mental situation because our training is hard enough to prepare for a big game. Mentally we have to do more.
"Every player has to self-motivate themselves, to make sure they are in the right position and be in the right way, so they can give good things for the team."