Forsberg, who has already missed four games, was first thought to have a simple strain of the groin, but he told the paper an MRI on Thursday revealed the tear.
"There was more damage there that any of us thought -- more than I thought," Forsberg told the paper.
Forsberg, the Flyers' leading scorer with 39 points, has been aggressively rehabbing in the past week, but after skating for 45 minutes felt worse than he'd expected and so he underwent the MRI exam.
The seven-time All-Star told the paper he did not believe the hard workouts to recover from what he thought was a strain caused the tear.
"I thought that if I didn't push it and then went into the game, it could be 10 times worse," Forsberg told the Inquirer. "The problem is, this is not game speed. It felt good for the longest time, but this is worse than we thought. I don't think I was too aggressive. Who knew it was a tear? It shouldn't be too long [for healing] now.
"I didn't think it could get worse; I just knew it was there. The MRI said it was a Grade I tear, just a little tear in the muscle. It should heal by itself. ... There is nothing you can do about it except let it heal and try not to tear it open again."
The Flyers host Calgary on Tuesday night, Edmonton on Thursday and Minnesota on Saturday. Forsberg said he would like to play Saturday, but is not optimistic.
"I don't know about this week," he told the paper. "I was so positive last week that I was going to play. I wanted to play last Saturday [in Nashville]. Whatever I say might be viewed more optimistically than the actual reality."