Onana was dropped for United's defeat at Newcastle on Sunday after being at fault for both goals in their 2-2 draw at Lyon for Thursday's Europa League quarterfinal first-leg.
Asked about the situation, Foster told talkSPORT: "You can't (recover that quickly). It's a horrible place to be and I've been in it so many times throughout my career.
"You're struggling mentally and the only thing you can think of is, 'Don't make a mistake.' And it's the worst way to go into games.
"If you can go into games with big confidence, positivity, knowing that you're going to help your team out, then you're on a great setting to start the game.
"But when you're thinking negatively and pessimistically, it's a really bad place to be, but I think Onana has been a little bit like that all season really.
"I see some of the mistakes that he's made and some of the saves that he makes, he seems to me like he's a little bit stressed when it comes to the big moments.
"Whether it's the big moment of making a save, whether it's a block or a parry, instead of being really sort of gentle and calm under pressure, he seems to be a little bit too stressed for me.
"So he's getting real blocky surfaces on things and everything seems to be popping out into that six-yard box, where you look at some of the top goalkeepers in the world and straightaway you think of someone like Emi Martinez or Alisson, or Thibaut Courtois.
"Them goalkeepers, when it comes to the real, big moments, it's that calmness under pressure, it's about being focussed but relaxed at the same time, and Onana just doesn't seem to be able to do that."