Leo has sustained a chipped bone in his ankle, which has been put in plaster.
It is rotten luck for the midfielder who has already missed two months of the campaign with a hamstring injury picked up after scoring twice in the game against Derby.
The busy midfielder has more than played his part in our recent run of form, including the clinical strike against Charlton in the FA Cup fourth round.
A downbeat Leo commented: "It is devastating to have suffered another long-term injury, especially when the team is playing well."
Boss George Graham lamented: "Leonhardsen has a broken bone in his ankle and will be out for up to six weeks. It is unfortunate but it continues this horrific situation with injuries at the club.
"We had a few blows the other day as well with Stephen Carr breaking down, Chris Perry, Darren Anderton picking up injuries and Iversen going home with a virus. It really has been horrific."
Our current injury list now consists of Ben Thatcher, Mauricio Taricco, Chris Armstrong and Carr (all groin/abdomen), Perry (calf), Anderton (Achilles) and Leonhardsen.
Oyvind Leonharsen's injury-blighted season has taken another turn for the worse with the news that he will be absent for six weeks following the X-rated challenge by Stockport skipper Mike Flynn.