Mainoo, 21, has rediscovered his best form since Michael Carrick replaced the sacked Ruben Amorim in January, starting every game available to him and helping United climb to third in the Premier League.
A verbal agreement on a new contract until 2031 has reportedly been reached.
Speaking on the Rio Ferdinand Presents podcast, Pogba, who like Mainoo came through the United academy before leaving for Juventus in 2012 seeking regular football, drew on his own experience when giving his assessment.
"It's difficult. It's really difficult. It's the way how he feels. If he feels that, if he's good with the manager, if the manager really trusts him — because he's going to have all the clubs behind him, this is for sure," Pogba said.
"But I don't know about his contract, because if he will be a free agent, it will be a different story, I think. A kid like this, that thinks he can play, and other people think he can play, his managers pushing him — that also puts this in his head and he's like, yeah, I should leave."
Pogba stopped short of advising a departure.
"Maybe it's not the right decision. And it's really hard for me to say he should leave when he's so loved by the people at United. He's such a great player, a big potential and he can do a great career at Manchester United also."
The former France international added: "I don't know, whatever decision he's going to take. I just want to see this kid play because it's enjoyable to see him play."