The 33-year old last featured in a 2-1 victory over Ipswich Town on 13 December when he was withdrawn with a heel problem.
Having worked his way back to fitness, the Blues frontman converted a 19th minute penalty for the development side before being taken off at half time along with fellow professional Jacques Maghoma.
“It’s good to be back out there,” smiled Donaldson. “It’s been a long time since I’ve played a bit of football like that.
“To get out there and play 45 minutes and to come through it is really pleasing.
“I tried to stay positive when I suffered the injury but when you get closer to a return and you can see the light at the end of the tunnel, it gets frustrating because you just want to be playing straight away.
“The physios and all the medical staff have played their part and kept me going so I’m looking forward to getting back out on the pitch again now.”
Donaldson showed flashes of the form which had made him a regular in the side during the first half of the season, whilst Maghoma, also returning from injury, was a constant threat throughout.
Having now returned to play his first 45 minutes, Blues’s second highest scorer is hoping to get back up to speed with ten games of the season remaining.
“I felt sharp at first but as the half wore on I could feel the legs going a bit but that’s normal,” he explained. “They've had me doing lots of conditioning work so I don’t feel too bad afterwards which is a positive.
“You just can’t replicate playing football matches so that 45 minutes will do me a whole lot of good.
“It’s the worst thing as a player having to sit there and watch your team mates play when you just want to be out there playing with them.
“It’s been a long, tough, three months but I think it’s been a learning curve as well because I’ve never been out for that long in my career.
“I’ve learned from it. You have to have a strong mind set and I think I’ve got that.
“It’s just about building the minutes up now and building the confidence that everything is fine with it.
“Really it’s just the match fitness that I need so 45 minutes is in the bank and hopefully I can go again soon.”