The Frenchman is out of contract next summer, which means he could leave on a cut price deal this summer.
However, Pogba has intimated he may extend his deal, depending on the club situation and the terms he is offered.
And Nani believes the World Cup winner must produce better performances to justify a huge new contract.
"At this moment, I see him as one of the important players at Man United," the Orlando City winger told Stats Perform News.
"But I don't know what he has on his mind, if he wants to leave or if all this news is making some problems in his head.
"I don't know if he has full concentration on the team, on the games, on the goals of the team.
"I saw him when he was struggling with some injuries and then he came back and started playing, he was improving his performance. He was doing well.
"He was helping the team, but we know Pogba can do much better, he can do much more because he is a top player in this position. He's not using all his strength, all his qualities.
"But if he wants to stay at the club, for sure he will perform much better very soon. He needs to continue to improve his game, his confidence and try to help the team in the way he can."