The Scotland international has been out after suffering from ulcerative colitis but, despite assistant manager Mike Phelan recently urging caution regarding any return to the highest level, he made an appearance as a second-half substitute at Pittodrie.
After being called up to train with Craig Levein's squad at international level, without actually being considered to play, the development is another welcome boost to the midfielder.
"I'm delighted and you saw the response he got, which was fantastic," Sir Alex admitted on MUTV afterwards. "Well done to the Aberdeen supporters who gave him a rousing reception and he deserves that for his contribution to Scotland's national team as he's been terrific for them.
"For Darren, it's great to be back and I hope it's the start for him. We don't know as we're all aware he's got this issue he needs to deal with and he's dealing with it very well at the moment. What we've seen tonight is encouraging for us."
Fletcher took over the captain's armband from Ryan Giggs just before the hour mark and contributed with his usual enthusiasm and eye for a quality pass in the 2-1 defeat.
"It's been a quiet progress for the boy," Sir Alex told MUTV before the game. "He joined in training a couple of weeks ago and it's one of these situations where you say: 'Let's see how he does' because he has had a really difficult time in the last year.
"Therefore, I wasn't really placing too much hope in it but let's see how he does.