Franzen had a subdural hematoma, more commonly known as a bruise inside his head, according to reports in the Detroit News and the Detroit Free Press.
Franzen had made headlines by leading the league with 12 playoff goals, but apparently was hurt during the Western Conference semifinal series against the Colorado Avalanche.
"I had a collection of blood, it was not in the brain, but between the skull and the brain," Franzen told the Free Press.
"It self-absorbed between two and three weeks," Franzen added. "It was a little bit scary. I couldn't do anything because that would have been dangerous."
Despite constant headaches, Franzen played in Game 1 of the conference finals against the Dallas Stars, but was scratched for the next six games, and did not return until Game 2 of the Stanley Cup finals against the Pittsburgh Penguins.
The team's explanation to the media was that Franzen was suffering from concussion-like symptoms. But an MRI exam had revealed the clot.
"It's not going to affect him long-term," Red Wings general manager Ken Holland said in the Detroit News. "Basically, in effect, it's a bruise. It's like when you get hit on a part of your body."
Franzen told the News that he had been concerned initially, but was told by doctors that the clot would not reoccur.
"I wasn't allowed to do anything and I listened to the doctors when they said that," Franzen said. "I didn't want to jeopardize anything. The doctors said it happens to football players and boxers quite often. It came from a hit but we don't know exactly when."