Tigers boss Nigel Pearson told BBC Radio Humberside: "It could be three or four weeks.
"He's been very good, and he's really disappointed, but there's not a lot you can do about it."
Barmby has proved to be an influential figure for the club this season, both on the pitch with his seven goals, and off the pitch where he assists with coaching.
Meanwhile, there is better news on the fitness of full-back Liam Rosenior, who has been struggling with a shoulder problem.
The former Fulham player will miss the game against Millwall.
"It's a bit of a shoulder problem," said Pearson. "But I don't think it's going to be too long a deal which is good.
"He's made quite good progress so hopefully he may be available for next week."
Rosenior, who joined Hull in October as a free agent, has made 22 appearances for the club so far this season.
Source: BBC Sport
Source: BBC Sport