New Zealander Reihana limped off just 10 minutes into Northampton's 28-7 Guinness Premiership defeat against Gloucester at Kingsholm.
Reihana briefly tried to continue before conceding defeat, but Saints head coach Paul Grayson said: "It could be a bad one.
"Fingers crossed, it might be nothing, but we will just have to wait and see."
The possible loss off Reihana would be a huge blow for Northampton, who have already lost centre John Clarke probably until Christmas because of a broken ankle.
If Reihana needs surgery, then it appears highly unlikely that he would be available when Northampton begin their European campaign next month.
The injury problems did not end there for Northampton, as props Tom Smith and Pat Barnard both failed to last the distance in a match that finished with uncontested scrums.
Grayson though, had no complaints about the result as Gloucester moved top of the Premiership thanks to tries from James Forrester, Andy Hazell and Jack Adams.
"Our lineout ball dried up in the second half, and we couldn't keep a foothold in their territory. We made too many basic errors," he added.
"I thought we were relatively comfortable at half time, but we ran out of steam.
"It was a very stop-start game, and we didn't play anything like the rugby we wanted to play. We were well below par."
Gloucester have recorded three wins and a draw in the opening month of Premiership action, and head coach Dean Ryan was quick to praise his players' efforts.
"I thought we played with tremendous commitment and I cannot help but be pleased. We did a great job defending against the Premiership's biggest attacking threat," he said.
Ryan also acclaimed another outstanding performance from openside flanker Hazell, who despite his rich vein of form is currently out of the England picture.
"Andy is one of the those players whereby we are not the same team when he isn't there," added Ryan.
"The same could be said for someone like Peter Buxton because they are very much the fabric of the team.
"I think the players deserve enormous credit for what they have done during the past few weeks."