The Highlanders started preparing for their second pre-season match yesterday but Donnelly injured his thumb in training during tackling practice, and the extent of the injury was to be assessed last night.
The worst-case scenario was a rupture of a tendon in his thumb and he could be out of action for up to six weeks.
Highlanders coach Glenn Moore said it would be disappointing to lose a player of the calibre of Donnelly.
The Highlanders play the Crusaders in a pre-season match at Alpine Energy Stadium in Timaru on Friday night.
Other injury problems have eased with outside backs Ben Smith and Fetu'u Vainikolo both coming back to full fitness and are possibly available for Friday night.
Smith is recovering from off-season surgery to an injured foot while Vainikolo is recovering from a hamstring tear.
Another outside back, Kenny Lynn, twisted an ankle in training last week and but is expected to be back to full fitness this week.
Moore said All Blacks Jimmy Cowan and Jamie Mackintosh should get some game time against the Crusaders.
All Black loose forward Adam Thomson is to see a specialist tomorrow to see if he will be able to play.
Thomson had an operation on a nagging thumb injury in the off-season but the thumb is healing nicely and he was showing no ill-effects of the injury at training yesterday.
Moore said the side had received plenty of benefits from its game against the Chiefs in Rotorua last Friday night.
The Chiefs pulled away in the final quarter to win 36-19.
Moore said there was lots to work on.
"Our set piece worked very well. We had a strong line-out and stole a bit of their ball. The scrum operated pretty well and was solid," Moore said.
"There are just structures to work on. Our defence structures need to be worked on - make sure our clean-out stuff shows more consistency."
Moore said the Highlanders had a strong opening 20min and worked hard throughout the game.
There were plenty of changes made by both sides, and that theme was bound to continue at Timaru.
The match against the Crusaders may be played in three 30min periods, Moore said, as various combinations are tried.
The Highlanders will hold an open day at Carisbrook on Saturday, from 2pm to 4pm.
There will be plenty of competitions, prizes, and games for everyone, with the Highlanders signing autographs and answering questions.
