The former Hoops keeper has been taken aback by the displays of the Englishman in Europe's premier club competition this season.
The 6ft 7in shot stopper has been immense in the Scottish champions' ties with Helsingborgs, Benfica and, most notably, Barcelona in recent weeks.
And Bonner is confident the ex-Newcastle man can rise to the occasion once again in his team's final Group G match against Spartak tomorrow night.
Neil Lennon's side need to better the result of Benfica, who travel to play Barcelona at the Nou Camp, to progress to the last 16 for only the third time.
Bonner stated: "I have been very impressed by Fraser this season and have no doubt that he will have another good evening against Spartak tomorrow night.
"He has done very well since first arriving at Celtic. His progress as a player in the last season or two has been nothing short of remarkable.
"It is all down to him. He has taken on board the information the coaches at Celtic have given him and has improved immensely. He deserves credit for that.
"Being a keeper at Celtic is different to being at keeper at most other clubs. You can have just one or two saves to make in a match sometimes. He has learned to deal with that."
He continued: "This is a massive game for Celtic and I am sure the tie with Spartak will be edgy with so much at stake for the club – a place in the knockout stages.
"But Fraser has already shown himself to be completely reliable in Europe, as well as domestically, and I am sure he will be able to cope with the enormity of the occasion and the atmosphere."
Former Republic of Ireland keeper Bonner was delighted when England manager Roy Hodgson called Forster up earlier this season.
And he thinks forcing his way into his national side in the future is not beyond the capabilities of the 24-year-old if he keeps on improving at Parkhead.
The former Norwich City player has one less rival for the No.1 jersey with the Carrow Road club's John Ruddy being ruled out for three months with a thigh injury.
"Bonner stated: "Fraser has got stiff competition for an England place with Joe Hart there. He plays for Manchester City and is in excellent form.
"What he has to do is be ready. He may be called upon to play in a friendly or Joe might get injured.
"He has to take his chance when he gets it.
"When he is called on he has to go in there and perform, he must be ready to do so – and prove that he is good enough to perform on an international stage.
"Performing so well with Celtic at such a high level in the Champions League has helped Fraser's chances and shown the manager he can handle the intensity of playing for your country."