The 33-year-old spent two years out of the game before joining the Scottish Premiership champions.
The 33-year-old was released by Sunderland in 2012 and proceeded to spend two years away from playing as he battled to win his fight against the problem that had plagued his time at the Stadium of Light.
He joined Celtic in 2014 and despite being out of the game for such a long period of time, he quickly established himself as the Hoops' number one.
He would make 55 appearances in his first season back and had made another 44 this term before Scotland's friendly with Denmark earlier this week allowed him to bring up his century. And he explained that the unique landmark is a very important one to him.
"In that first season, if I'd played over 10 games, I'd have been happy with that. But I ended up playing 55, which was beyond anything I had imagined after coming back," he told the Mail.
"I didn't think it was possible - but it was. So it was a big milestone for me to reach and I'm glad to have carried on.
"Probably after six or seven games with Celtic, I realised I was in pretty good shape, and that my reflexes were still what they had been before. I didn't feel weaker in any way - in fact I probably felt better than I did previously, when I was maybe carrying injuries and trying to get through games.
"To come back, and to get myself really fit … I realised soon after I started playing again that I was okay, and that I'd be capable of more than 10 games in my first season back.
"Then I wanted to get as many as I possibly could. When milestones get closer they do start to enter into your head. I think the other night was my 100th game since my comeback from injury, so that was special for me as well, a bit of a milestone."
While the former Hearts stopper would have been forgiven for being rusty and needing time to adjust to the demands of first-team action following his return, his performances have arguably been as good as they have ever been.
He helped Ronny Deila's men keep the strongest defensive record in the division last season - as they conceded just 17 goals all year in the league - and they appear to be well on course to achieve that feat once again this term.
And with his injury problems hopefully in the past, he can now concentrate on enjoying the peak years of a goalkeeper's career and helping the Bhoys win plenty more silverware.