United ended their Champions League group-stage campaign with a 2-1 defeat to Valencia that saw them miss the chance to finish top of Group H above Juventus, who slumped to a surprise loss at Young Boys.
United take on Premier League leaders Liverpool at Anfield on Renault Super Sunday, and former Reds forward Bellamy has cast doubt over Pogba's participation in the fixture.
"Pogba was poor again tonight," he told The Debate.
"Mourinho isn't getting the best out of Pogba, that's clear. There is a lack of interest from Pogba. His performance in Valencia was concerning.
"I'm not sure if he is as good as a lot of people make out.
"There's a massive game on Sunday, Liverpool versus Manchester United is the biggest game in the Premier League between the teams who have won the most trophies, an incredible fixture.
"If you're Mourinho, after Pogba's performance today, it will be very difficult to say he has to play, he has to leave him out again."
Pogba has failed to start United's last two Premier League games, starting the 2-2 draw with Arsenal on the bench, while he was an unused substitute in Saturday's win over Fulham.
Bellamy believes the France international's display at the Mestalla was evidence that he is going backwards.
"Usually, these kind of performances convince the manager he would be of use in games like the one on Sunday, but going off the back of the performance in Valencia that is not the case," he added.
"We get reminded that Pogba won the World Cup every time we talk about him, but for a player that is used to playing in big games, he doesn't look like he's improving, he's going backwards."