Foster aired his grievances on his Fozcast podcast, drawing an unfavourable comparison between Garnacho's demeanour after the final and that of Tottenham goal scorer Brennan Johnson.
Speaking on his Fozcast podcast, he said: "I watched the Europa League final and you've got Brennan Johnson doing his interview after the game. He just looks like a lovely, hardworking, honest kid.
"You can tell with the way he's speaking that he's a team player. He might not have the ability of Garnacho...but he'll outwork him, I guarantee you.
"The full-time whistle goes and he sits down on the floor. He doesn't try to interact with any of his teammates or try to console them. It's all about him, him, him, him. Brennan Johnson is a team player. Give me him ten times out of ten. Garnacho? No chance."
The Red Devils seem to be wasting no time in their squad overhaul, with Matheus Cunha tipped to be the first new face at Old Trafford, potentially signing a deal until 2030 with an option for an extra year.
United's readiness to meet the £62.5m buyout clause for the Wolves striker indicates their commitment to bolstering their attack. But Foster has reservations about the move, casting doubt on Cunha's character.
He said: "It's more of the same. They need to get back to basics. I'd rather, as a United fan, have good, honest, hardworking lads that might not have the ability of Cunha or Garnacho, but I know they're going to do it professionally and they're going to be there to stand up and be counted when needed.
"I think United failing to get Champions League football and the money that brings just opens them up to this mercenary level of player signing. You're going to get problems down the line."
Meanwhile, Cunha has penned an emotional message to Wolves supporters, seemingly hinting at his exit from the club. "Wolves, what can I say to you?" he said.
"What an incredible season. We didn't get to where you deserve to be, but we did everything we could to make you proud. I love these guys.
"Individually, it was the best season of my life. All of this was only possible because of all the dedication and love I have for this club. I became the Brazilian with the most goals in a Premier League season, along with @roberto_firmino and @gabriel.martinelli."
Cunha added: "I made mistakes and got things right, but always because I lived this club so much. All I ever wanted was to do the impossible for Wolves. Another one is over. And thank you all for all the affection."