Forster has excelled since his return to Celtic on loan this season, having left back in 2014 in a £10 million move to Southampton.
The 31-year-old keeper remains under contract at the English Premier League club until 2022 and his basic salary of around £70,000 presents an obvious obstacle to Celtic manager Neil Lennon’s hopes of re-signing him on a permanent basis.
But Bonner feels the obvious pleasure Forster derives from wearing the Celtic jersey could motivate the big Englishman to make a financial sacrifice if a deal can be agreed with Southampton.
“The grass is not always greener somewhere else,” said Bonner. “Now, of course, Fraser is on big money with Southampton. It’s incredible money players get down south and it’s hard to know if Celtic could even afford to take him for half his money.
“Possibly not, but he seems to be happy at Celtic. He’s made his money and is this the place for him to spend the rest of his career? To finish his playing days at a club which he enjoys?”.
Bonner, who made more first team appearances than any other goalkeeper in Celtic’s history, has been hugely impressed by Forster over the past few months.
“Fraser makes the big saves,” added Bonner. “He’s a big personality now that he’s come into the team.
“I talked with Neil Lennon in the past about what does he want as a goalkeeper and he likes a big and imposing guy.
“Fraser is an immense presence. He doesn’t come out for many crosses given his size, but I think he has really improved his work with his feet.
“Since he’s been down in England, he seems to have learned and got better with that, even if he’s never going to be brilliant at it
“ But he makes the big saves at the big times and I admit he’s actually done even better than I thought he would since coming back.
“When he signed for Celtic the first time around, he was a big goalkeeper, but seemed a little bit sluggish and slow. He seemed to do a lot of work on his speed and movement and he turned out to be very good.
“Having been away from Celtic for such a long time and missed a lot of football during the latter stages of his time at Southampton, I wondered if he might have lost a bit of that speed with his lay-offs.
“But he’s still got it and that’s the critical factor for him because he is such a big guy. Yes he’s tall, but that’s not enough for a keeper, you need momentum and quickness . All of that stuff is part of the package.
“I think it’s also very important that he enjoys being at Celtic. He feels that people respect him and value him.”
Forster has helped Celtic rack up a current run of 10 straight Premiership wins which sees them five points clear of Rangers at the top of the table, having played a game more than their Old Firm rivals.
Bonner has also praised the contribution of central defensive duo Christopher Jullien and Kristoffer Ajer who have quickly formed a good relationship with Forster.
“The reaction of Jullien, Ajer and Forster at the full-time whistle in the game against Hearts last midweek was very interestin,” said Bonner.
“Celtic coasted to the win in the end, but they clenched fists at full-time. It showed me that they take pride in keeping the ball out of the net.
“Of course, you have to be good in other parts of the game as defenders, but the two centre-backs look as if they enjoy defending.
“Ajer is a young man and he can get caught up in things. He needs to calm down a bit, but Jullien is there and he looks a real defender.
“The reaction to the clean sheet at the full-time whistle is what I like to see and the first thing I look for after a game. You could see the passion in them to defend the goal and get a shut out.
“I also look at how they react when they lose a goal and they are. Fraser went through a real spell the last time he was at Celtic when he set a record for minutes without losing a goal. That’s the type of thing he has in his memory and he wants these things more.”
Forster’s success has forced Craig Gordon to consider his future at Celtic as he bids to regain his place in the Scotland squad but Bonner hopes the veteran keeper sticks around.
“It’s difficult for Craig,” said the Irish legend. “He’s done exceptionally well at Celtic. Even when he was brought back for the Cluj game recently, he made some excellent saves and you need a top-quality second-choice goalkeeper.
“You need to be ready to play when called upon in that situation and he proved in Romania that he is.
“Craig is 36. His career got back on track at Celtic, he won medals and made money. If it was me and I was in his shoes, I would stay.
“I think his next step is getting into coaching and thinking about the next part of his career, so where else would you go where you are going to be involved in European football and winning trophies?.
“He could still play for another couple of years and who knows what can happen? He could be back in the Celtic team at the drop of a hat.
“I understand the international situation is a big thing for him and, if he’s not playing at Celtic, he probably won’t play for Scotland. But Craig would always be in the squad because he’s good enough and going to the Euros could be a lovely end to his career.”