Fletcher came on as a second-half substitute against Aberdeen on Tuesday as he continues his comeback from ulcerative colitis.
Ferguson said: "I'm delighted and you saw the response he got, which was fantastic.
"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."
A United XI lost 2-1 to Aberdeen in a testimonial match for Dons player Neil Simpson.