The Ospreys expect Henson to be sidelined for between four to six weeks, meaning he will miss remaining Heineken Cup pool games against Ulster (twice), Gloucester and Bourgoin.
Henson though, should be available for Wales' RBS 6 Nations Championship opener against World Cup finalists England at Twickenham on February 2.
Henson made an impressive return to the Test arena last month - only his second international start in a year - despite Wales' 34-12 defeat against world champions South Africa.
But he also courted the wrong type of headlines earlier this week following alleged rowdy behaviour by Henson and three friends on a Cardiff-bound train from Reading.
Just hours earlier, Henson scored all the Ospreys' points in their 19-8 victory over Quins.
Assessing Henson's latest injury blow, Ospreys team doctor Rhydian Lewis said: "From reviewing the match video, it appears that an opposition boot accidentally came into contact with his hand and Gavin can clearly be seen holding his left hand as he gets back to his feet.
"Gavin was able to complete the game, albeit in some pain, and he was checked out immediately after the final whistle by our medical team, who diagnosed a suspected fracture of the fourth metacarpal in his left hand.
"We then secured the earliest available appointment for him to undergo an x-ray on Monday, when our initial fears were confirmed.
"Gavin has now had his left hand set in a lightweight cast and, while he will be able to continue with his fitness training, it is expected that he will be unavailable for selection for a period of four to six weeks while the bone heals."
Ospreys rugby director Kevin Hopkins added: "Both Gavin and our medical staff have worked hard in an effort to get him on the pitch against Ulster tomorrow (Thursday) night.
"But in the end we have decided that Gavin's long-term wellbeing is best served by him getting the hand properly sorted over the next few weeks.
"Gavin is an excellent rugby professional and no-one works harder at their skills and conditioning than he does."
Henson's injury is an immediate blow for new Wales coach Warren Gatland.
Although current indications are that Henson will be fit and available in time for the England clash, he is set to sit out several Heineken Cup and Magners League games.
And that lack of matches could prove a concern to Gatland in the build-up for Twickenham, a ground where Wales have not won since 1988.