The English shot-stopper missed the 4-1 Parkhead win over Hearts and will again sit out tonight as he continues his recovery from a back injury.
Forster is not the only injury doubt ahead of the United clash, but he is the one causing most concern for boss Neil Lennon as he prepares his side for a massive few weeks.
Parkhead assistant Johan Mjallby said: "Kris Commons and Adam Matthews are both doubtful, although we will leave it as late as possible to decide on them.
"James Forrest and Paddy McCourt are still injured and they won't be in the squad. Fraser has a stiff back, but that's more of a precaution to leave him out. We want to make sure he's raring to go as soon as possible because there are some big games coming up; the semi-final against St Mirren, the Scottish Cup against Raith Rovers and the big one against Juventus. So we have to make sure Fraser is 100% fit.
"Nothing really came up on the scan to be worried about which is great. He was feeling it a wee bit in Marbella and then on the flight home.
"That didn't really help as he's very tall. A cramped plane? Yeah, maybe. But he was feeling it over there so we can't just blame the flight.
"But we're in a good position as we have two fine goalkeepers.
"It's really handy that we have such a good, astute keeper in Lukasz Zaluska, who can be there for us. We're in a privileged position."