Mainoo, who's currently earning a modest £20,000 a week, might start feeling disgruntled especially when looking around the squad who earn ten times his wage. Talks are reportedly ongoing between Mainoo's agents and United chiefs about a contract extension but Foster has sent a warning to the club stating that other top teams would gladly pay Mainoo what he is worth.
"It must really annoy Kobbie Mainoo because he must be looking around at some of the players that are on crazy wages that are nowhere near as good as him," said the former Wrexham goalkeeper on his Fozcast podcast.
"They don't put in any of the performances that he puts in, and they're earning three, four, five or even six times the amount he is. I thought that after the Euros, he was going to go on to be a world-beater this season."
"I still think there's a world beater there as well and it's probably mental fatigue or a bit of burnout. I think we will see an absolute player."
Reports suggest Mainoo's representatives reportedly pushing for a hefty £150,000-a-week wage from United but with the club currently trying to cut costs under owner Sir Jim Ratcliffe this may not be feasible. Foster argued that rivals Chelsea would be willing to fork out the money needed to tempt him away from the club which will worry United fans who want to see the club bounce back under manager Ruben Amorim not lose one of their best young talents.
"He's not in any position right now because of circumstances to go making demands. He's still got three years left on his contract with an option.
"If he was at Chelsea, he'd be on £150,000 per week so that's his argument. But then the club come at it from their perspective and go, 'Well, you've still got three years left on your contract.'".