The Manchester United midfielder was out of action for almost a year with a chronic bowel complaint.
But, despite what he has gone through with the illness over a number of gruelling months, Fletcher insists he is ready for action if called upon by Craig Levein.
Speaking ahead of Friday night's clash in Cardiff, he said: "I feel ready to play both matches. I feel fit, I feel strong and I feel ready for the games. It's great to be back.
"It's been a while and I'm just desperate for the game to start. Hopefully I'm playing and I can do my part in getting a good result for Scotland."
Fletcher claims he never gave up hope of pulling on the dark blue jersey again.
He added: "Not at all. I always believed I would be back and fortunately I'm here today. It proves that I always had the belief that one day I would be back and helping the lads qualify for a major competition."
Namesake Steven Fletcher has also made a dramatic return to the squad after almost two years after resolving his differences with Levein.
But Darren Fletcher insists any success against Wales will not just be down to their respective comebacks.
He said: "If we beat Wales on Friday it won't just be because me and Steven are named in the team.
"It will need a good team effort. If it can be a boost to the rest of the players that's great. You look for any small margin you can.
"But knowing the players, they will be ready for this game irrespective of whether me and Steven are in the team.
"We'll be looking to win the match and whoever is playing will give their all and hopefully we can get a positive result for Scotland."
Scotland are seeking a win after draws against Serbia and Macedonia at Hampden, while Wales need a positive response following a shock 6-1 thrashing in Serbia.
Fletcher said: "Wales are a team full of players playing in the Premier League in England, with the ability to be a really good side.
"I know they had a difficult result away to Serbia. People see that as them being low in confidence and they might well be but you can also look at it the other way and say there's a real determination about them to prove that they are a good side and I think that's what we will see tomorrow [Friday] from Wales.
"They will be full of determination to prove that was a fluke result so we've got to be very wary of that.
"I was surprised actually because that's not a true reflection of Wales or Serbia to be perfectly honest.
"Having played against Serbia, we know they are a decent side but I think for Wales to lose 6-1, it wasn't a fair reflection and I'm sure they will be desperate to show that."