Jagr, who injured his right hamstring Monday night in a 2-1 loss at Tampa Bay, returned to Pittsburgh on Tuesday. He was placed on injured reserve and is out indefinitely.
"It could be 10 days it could be a month plus," Penguins coach Herb Brooks said before the Penguins played the New York Rangers.
"It's still too early to know until he sees some doctors back home and gets a little different feel," Brooks said. "So that obviously takes a big chunk out of our lineup."
Jagr, the NHL's leading scorer with 85 points, was held scoreless for a season-high third consecutive game. He said he heard his hamstring "crack" when he got tangled with the Lightning's Todd Warriner late in the second period.
"His production has been well, you know the stats," Brooks said of Jagr's drought.
The Penguins are in seventh place in the Eastern Conference and are locked in a fight for the playoffs with six teams.
At 6-foot-2 and 234 pounds, Jagr combines good size and speed that usually allows him to avoid big hits. However, against the Lightning, last season's MVP took two hard shots in the first period.
"He's been playing hurt and trying work his way through it," Brooks said. "At the same time he's being mugged half the nights. Last night, I thought I was at the Calgary Stampede."
In January, Jagr missed four games including a home contest against the Rangers with an injured stomach muscle. The injury that was thought to keep him out up to two weeks, and prevent him from playing in the All-Star game, sidelined the Penguins captain for about a week.
Jagr has also been bothered by a hand injury after being slashed just prior to the All-Star break. Earlier in the season, a thigh muscle injury forced him to miss another game.
The 28-year-old right wing is second to Florida's Pavel Bure with 37 goals and was held without a shot in 14 shifts, totaling 16 minutes Monday night.
The Penguins recalled forward Robert Dome from Wilkes-Barre/Scranton on Tuesday to fill Jagr's roster spot.
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