Was it relief that his two-year fight to get his playing career back after injury threatened to end it prematurely had paid off?
Or was it delight that he was not only back, but with the top club in the country - and with a two-year deal?
I would imagine it was a mixture of both, and I wish him the very best of luck as Part II of his career begins.
I can empathise with Craig in so much as I found myself in a similar situation when I was also 29.
A combination of the detached retina I had suffered a year or two before and a serious knee injury meant I was told I had to quit playing, the only job I had ever known.
Chelsea were good enough to give me a chance at coaching the kids, and that opened up a new chapter in my working life.
But, no matter if you manage to get a full career playing until age forces you to quit, or your time on the pitch is brought to an abrupt end because of injury, it is a day in your life everyone dreads.
Craig's knee injury, for which he has had two operations, looked to have brought him to that moment.
And, in the two years since he last played a competitive game, he has had plenty of time to come to terms with accepting it was all over.
But, to his credit, at 31, he refused to give up on his dream of playing again. And, for that, we should all applaud him.
Craig has spoken about not wanting to find himself in later life asking: What if I had just battled through the pain?
I can relate to that, though my circumstances meant the decision to keep trying to play on was taken out of my hands.
I had somehow managed to continue while only really able to see out of one eye, the result of a tennis accident.
That was fine when I was operating at the back. But in midfield it did make life very difficult because you could not always keep the game in front of you and in the line of your good eye.
That said, I did manage to win a man-of-the-match award in one game when I was in the middle of the park, which still makes me smile.
But, when my knee problem was added to the reduced vision, it was the specialist who told me I couldn't continue to put myself through what I was having to do to continue playing.
Even Frank Sinatra eventually had to call time on his comebacks. But, it was still a wrench to let go of my playing career.
In my day, planning for the future was never a consideration until you got past 30 and started to realise the end was on the horizon.
Nowadays, I don't think players have to think about their life after football because it pays so well and there are insurance policies for those who have to quit early.
So, it tells you everything about Craig's character that he was not content to sit back and accept it was all over.
I'm not really surprised, because that's the kind of guy he has always been.
I remember watching him play one of his very first games for Hearts against Livingston. Despite the fact he was very young, he had a confidence and self-assurance which marked him out as one to watch.
After his £9million move to Sunderland, he was developing into one of the best keepers this country has ever produced.
Now, he will feel as though he has been given a second career - and the bonus is that it is with such a big club as Celtic. Can he get back to the level he was before? Craig, like the rest of us, will soon find out.
Unlike outfield players who have been out of action for so long, his fitness will not be the issue.
As he has identified, it will be his decision-making, and it's only in competitive games he will find out how much work needs to be done to get that up to the required level.
I am sure there will be some apprehension on his part, not about his injury - which I am sure has been tested to the nth degree - but about playing in front of a big crowd again.
That will be his biggest test, and it is a mental rather than physical thing. Craig will know all eyes will be on him to see how he performs.
But, the past two years will have told him what condition the injury is in, as will the strenuous medical he was given my Celtic.
I remember that, when you have had a serious injury, initially you are looking for every twinge as you train.
But when you take to the field, those thoughts disappear.
Every signing is a gamble for a club. But, despite his medical CV, Craig's signing is surely a win win for Celtic.
If he doesn't manage to get back to the heights he hit before, they have lost very little because what was a £9m keeper came to them on a free.
If Craig can reproduce that form, what an asset they have acquired. It could be a great move for Celtic, and for Craig.
Certainly, no-one can wish him anything but the best of luck because he has shown such bravery to give it a go.
But I am sure he is not looking for sympathy or an easy ride.
He will have even more hunger to succeed than he had first time around when he was a fresh-faced kid with dreams and ambitions.
And, at 31, there are very many more years to enjoy, provided his knee - and every other part of his body - holds out.