The Scotland openside flanker has been at the wrong end of a catalogue of injuries that have restricted him to a handful of matches over the past three seasons.
However, now back to full fitness, Donnie decided it was time for a new challenge - joining Edinburgh on a part-time basis to help with coaching strength and conditioning.
Donnie told The Times: “It was a very difficult decision to make and I changed my mind about it probably more than twice a day for the last couple of months.
“I am very happy about it, though. I am certain it is the right decision. I’m excited about what I’m about to embark on, going back to university to study sport science, with a view to strength and conditioning coaching – an area that I have developed a particular interest in over the last few years, when I have been spending a lot of time in the gym.”
Donnie's recent injuries include damage to both knees, requiring surgery, and lengthy rehabilitation in both cases, a groin-muscle tear, a broken foot and a hernia - a reminder of the risks inherent in playing and training for rugby.
“When I came back from the first knee operation, I comforted myself with the thought that it could not happen again, that lightning couldn’t strike twice – and of course it did,” he said. “I’m sure people will say I’ve retired too soon but I’ve got to start looking to the future.
“I decided when I got injured that I would get back to playing again and be fit and available for selection and it was fantastic to get a couple of games in at the end of the season. It meant I went out as a fit player rather than somebody who was forced out by injury. Physically I am in good shape at the moment.”
The other frustration for Donnie is that he is leaving just as the team look as though they could be about to turn the corner.
“It is fantastic to see the guys doing so well,” he said. “I have seen how things have evolved here, the way things have started to go, and next season could be fantastic.
“The core of the squad is not the group of young guys who a couple of seasons ago were fresh-faced newcomers – they are now ready to take things forward.
“The back-row strength in depth in Scotland is fantastic. Just look at the results the guys have had – a convincing win in Ulster backed up by beating Munster, the European Cup finalists, in their own backyard. That kind of thing used to happen occasionally, now it is happening consistently.”