The big Englishman broke Bonner's post-war record of eight successive league clean sheets for the club in the win over Hibs last weekend.
And he extended it to 11 by holding St Mirren at bay in yesterday's 1-0 league win at Parkhead.
Bonner, who played 641 games for Celtic, is convinced Forster's best form is still to come. "He will continue to improve and mature," he said.
"He's had two cracks of the Champions League now. This year was more disappointing than last year when he was the guy who was responsible for us getting to the last 16."
Bonner says Forster's efforts are under-rated when Celtic dominate domestic matches.
He told the club's website: "I watched the Hibs game last Sunday and he pulled off a remarkable save, a fantastic save to help them make this achievement.
"I used to hate it when I was a goalkeeper and people said to me coming off the pitch, 'Oh, you had nothing to do today'.
"It's not about having nothing to do - it's about keeping your concentration … I have seen goalkeepers getting plenty things to do but losing five goals and getting man of the match.
"It's a completely different way of playing.
"You might have five or six key things to do in a match and that's it. But you must have full concentration.
"I think he's doing that now and the defenders seem to very comfortable with him."