Lukaku managed to break his scoring drought in victory over Newcastle in a game which saw Ibrahimovic make his celebrated return.
Carragher told Sky Sports: "Lukaku's goal could be the most important in that 4-1 victory, because, if we are talking about Zlatan and the effect he is going to have on Manchester United – and it is brilliant for United to have a player of that quality – but when he comes on, it's hilarious, he tells Lukaku to get on the right wing and Lukaku does it. Zlatan is not going to do that job, and Lukaku doesn't want to do that either.
"The goal, the confidence it will give Lukaku having not scored in a while, and the big man's back calling himself a lion. I don't know if he (Zlatan) will have too much mentally and if it will effect Lukaku.
"You can imagine Zlatan in training, telling Lukaku to get on the right wing. Sooner or later, Lukaku is going to have to tell him, no you get on the right wing, I'm the £90m centre forward. I don't think they can play together."