United completed the signing of Buchanan on a free transfer after he was released from Kaizer Chiefs at the end of last season.
The 27-year-old endured frustrating times at Naturena, where he managed only a handful of appearances, and has been on the road to redemption at The Team of Choice.
However, the club on Wednesday raised fears that the former Bloemfontein Celtic playmaker could be ruled out for the rest of the year with a knee injury similar to the one he suffered at the Free State-based outfit in 2016.
"It's a blow for us to lose him [Buchanan]. It happened at training, he will have a scan today to determine the extent of the injury," said coach Fadlu Davids in a club statement.
"But it's definitely an ACL (anterior cruciate ligament), maybe not a complete rupture, either grade 2 or 3, which will put him out for a couple of months.”
The news comes as a blow to Buchanan, who had started the season well with three starts from Maritzburg's four games thus far.