Donnelly (28) will have surgery tomorrow to insert a pin into an ankle that has been bothering him all season.
The recovery period is six weeks, so the outstanding second-rower may not play again this season for the Highlanders, who have seven weeks of competition (including a bye week) remaining.
"I've been playing with a sore ankle for most of the season," Donnelly said yesterday.
"It wasn't as bad at the start. I could get through it. But it sort of got to a point where it was just too sore to run properly.
"The healing time is about the same if you let it heal naturally as it would be if you put a pin in it.
So the best option is to put the pin in, so it will definitely heal."
Donnelly's aerial prowess, work rate and experience were recognised at the top level last year when he made his test debut against the Wallabies and quickly cemented his place in the All Blacks.
He should be fully fit by the time the international season starts but may find it difficult to get in the squad without match-fitness and games under his belt.
"It's always tough, when you're not playing Super 14, to harbour any hope of getting back in the All Blacks.
"Probably, the NPC will be my next outing and we'll go from there."
Having Donnelly, who has 62 caps for Otago over eight seasons, and fellow Highlanders lock Hayden Triggs will be a boost for new Otago coach Phil Mooney, especially with the Ranfurly Shield challenge against Southland on August 7.
Donnelly has had major shoulder issues in the past but his body has been relatively kind to him in recent seasons.
That is why he is dealing with his ankle injury with maturity.
"It's always frustrating.
Any injury annoys the hell out of you," he said.
"But I suppose the good thing is that I had a pain in my ankle and I didn't know what it was.
Now I do know, and I can get it sorted out.
Six weeks is pretty minor in the scheme of things."
The Highlanders still have plenty of height left in the second row, in the form of Triggs, rising Southland star Josh Bekhuis and intriguing project Joe Tuineau.
They do not have a specialist lock in their wider training squad, but they are short of general forward cover with Donnelly out and No 8 Nasi Manu still injured.
That could lead to a call-up for wider squad member Hoani Matenga, who is a blindside flanker who can play lock, or the Highlanders could approach another franchise for a lock - someone like Andrew van der Heijden (Blues) or Api Naikatini (Hurricanes).
Highlanders coach Glenn Moore said he was unlikely to call for an extra player at this stage, and he hoped Donnelly could play a game or two at the end of the season.
Donnelly is confident the Highlanders, hoping to build some momentum after a 39-29 win against the Lions, will perform well over the second half of the season.
"Hopefully, that win was the turning point and the guys can win a few more games."
