MacDonald was carried off on a stretcher in the dying minutes of the first half at Dean Court and Howe afterwards predicted he would be sidelined for up to two months.
The Cherries boss said: "Initially we thought Shaun had broken his ankle, but we're optimistic it may only be a bad sprain.
"He'll have an X-ray and a scan, but whatever the situation he'll probably be out for a couple of months. The injury occurred when he blocked a cross.
"He's a massive player to lose, not only a great player but a great character as well. He's a calming influence in midfield. An unsung hero in our team - and we really missed him in the second half."
MacDonald's injury had an obvious impact on both sets of players and Lewis Grabban's sixth goal in as many matches was enough to settle a subdued contest in Bournemouth's favour.
The win means Howe has not lost in 13 games since returning from Burnley in mid-October and hauls the Cherries to within two points the League One play-off places.
Howe added: "In the first half I thought we were excellent, but were a bit below that level in the second.
"All in all protecting a 1-0 lead against a team we've not done particularly well against of late was a good result."
- Sports & Recreation