CINCINNATI -- Reds star Ken Griffey Jr. will miss at least 3-to-6 weeks after injuring his troublesome right knee in a rundown during Sunday's game against Montreal.

ESPN'S TAKE

I feel badly for Ken Griffey Jr. He was frustrated last season with the hamstring injury. I'm sure he worked hard over the winter to get ready for a good season -- and now he has the knee injury. After going through a season like last year, when he missed 51 games, I'm sure the only thing on his mind this year was going out and playing. But injuries are a part of the game you just can't predict. Griffey wanted to show people that he is still the same Griffey -- laughing, joking, having a good time, and playing baseball. I was looking for him to get back to being Ken Griffey. The game is better when players like Griffey have monster seasons. He is a huge cog in the Reds' lineup, with players like Barry Larkin, Sean Casey and Adam Dunn hitting around him. They will have to carry the offense until he returns. -- Tony Gwynn

Griffey partially tore the patella tendon and partially dislocated his right kneecap. Cincinnati placed the All-Star center fielder on the 15-day disabled list after a 10-inning, 6-5 win over the Expos.

"It's a serious injury," team doctor Timothy Kremchek said after Griffey had an MRI exam.

The Reds will try to treat Griffey with rest and rehabilitation.

Kremchek said there was an "outside chance" Griffey will need an operation. If surgery is required, Griffey would likely be out for the rest of the season, the doctor said.

Ruben Mateo was called up from Triple-A Louisville to take Griffey's spot on the 25-man roster. Mateo was batting .364 (4-for-11) with a run scored and two doubles in three games with Louisville.

Mateo was originally in the Reds' starting lineup Monday in right field against the Pirates, but was scratched with a head cold and replaced by Adam Dunn. Juan Encarnacion replaced Griffey in center field.

Griffey, who was bothered during spring training by tendinitis in the same knee, was hurt as he was being tagged by Expos third baseman Chris Truby in the seventh inning. Griffey had tried to score from third base on a grounder and was trapped in a rundown.

Griffey went down holding his knee and was briefly attended to by Cincinnati medical personnel before limping gingerly off the field.

Last year, Griffey missed most of the first three months because of a partially torn left hamstring.

Earlier in the game, Griffey hit his 461st career home run.

Griffey spent his first 11 seasons with the Seattle Mariners before forcing a trade to Cincinnati.