Henry incurred the problem in a challenge with Stephen Warnock in Saturday's 1-0 defeat away to Leeds and a subsequent scan has revealed ligament damage in his left knee.
Lions manager Kenny Jackett confirmed the bad news after Tuesday night's 2-0 defeat at home to Wolves.
"Henry will be out for the rest of the season. He has a knee ligament injury due to the tackle on Saturday," Jackett told the News At Den website.
"We've had the results of a scan. It doesn't really surprise me given the severity of the tackle. We did the scan once the swelling on his knee had settled down and his ligament is stretched.
"It does look like that is his season done."
Jackett also confirmed he will assess the situation before deciding whether to dip into the loan market to secure cover for Henry.
"I'll think about that. The first thing is the players I have," he added when asked about a possible loan move.
"They have to put in a better performance than they did against Wolves, that's imperative.
"Whether we need anybody from the outside will be an assessment of the response I get from the players."