And the former Celtic centre-half thinks securing the 31-year-old permanently would be Neil Lennon's best signing since his return as manager.

Forster has impressed on his season-long loan from Southampton.

"When you look at England's current number one, Jordan Pickford, Forster's probably in better form," Stubbs said.

"So he just needs to keep doing what he's been doing, performing very well, and if he does then I'll be very surprised if a call-up is not coming for him very shortly."

Forster joined Celtic on loan from Newcastle United in 2010 during Lennon's first stint as manager before making the move permanent two years later.

He was sold to Southampton for a fee of around £10m in 2014 but lost his place at the start of 2018, only made one appearance the following season and was down to third choice at St Mary's Stadium before Lennon signed him for a second time.

But Forster, who earned the last of his six international caps in 2016, is contracted to the English Premier League club until 2022.

"If Neil can do it, I think it will be the biggest signing he's made since he's been back at Celtic," Stubbs told BBC Scotland when asked about the possibility of a permanent move back to Celtic Park.

"Yesterday's performance showed how important it is to get him signed as quickly as possible because, at times, it was a one-man shut-out against Rangers.

"The saves he made were nothing short of incredible at times."

Among Forster's Hampden Park saves was a penalty stop from Alfredo Morelos and former Rangers midfielder Alex Rae thinks Celtic should act quickly before the goalkeeper's transfer value increases further.

"The more he puts in performances like that, the price will go back up," Rae said. "I know they have two good keepers down there, but I wouldn't be surprised if Southampton tried to recall him, because they will be monitoring his progress.

"If I was Celtic, you would probably be trying to sign him sooner rather than later."