When the Phillies signed Ryan Howard to a five-year, $125 million deal a year and a half before he was eligible for free agency, Ruben Amaro Jr. bet on Howard's remaining healthy and productive well into his 30s. Less than two years into the mega-contract, a surgeon will cut into Howard's left leg for the second time.

Howard tore a meniscus in his left knee, the team said Monday. He will miss six to eight weeks from the date of his surgery, which has yet to be determined. His season could be over, depending on how quickly he can recover and whether the Phillies are still in contention come September.

"We can only speculate the length of how long it will take to get him to rehab," Amaro said. "We'll shoot for the conservative one and hope he comes back faster."

The Phillies general manager compared Howard's injury to that of Erik Kratz, who began a rehab assignment with triple-A Lehigh Valley on Monday. Kratz underwent knee surgery June 12 and could return by this weekend. He is well ahead of schedule.

But Howard, 33, has a history of trouble with his left leg. Surgery was needed to repair a ruptured Achilles tendon in October 2011. He has recently complained of left foot problems. Amaro said he was totally unaware of such issues.

Howard first appeared on the team's internal medical report this season about May 4. He received a cortisone injection May 19. Phillies head athletic trainer Scott Sheridan said then that inflammation revealed in an MRI exam "concerned us." Howard returned to action May 21 before succumbing on Saturday.

The situation is eerily similar to when Howard tore his Achilles. He was injected with cortisone on Sept. 18, 2011, after weeks of pain. He collapsed on the field on Oct. 7. Major surgery followed.

This injury is far less serious, although both Amaro and Sheridan have conceded that it could be connected in some way to the Achilles. Amaro said an MRI in May showed "fraying" of the meniscus, but not a tear. An MRI on Monday revealed the tear.

"We have to take care of it," Amaro said. He added, "Everybody has wear and tear when they get older."

Not every recovery is as smooth as Kratz's. Cincinnati first baseman Joey Votto injured his knee in late June last season, attempted to play on it for two weeks, and then opted for surgery to repair the torn meniscus. Votto missed the next 10 weeks.

Howard is under contract through 2016. He has played in 151 games since the Achilles injury and hit 25 home runs with a .244 batting average and a .752 on-base-plus-slugging percentage. Amaro said he would not pursue an upgrade at first base in Howard's absence.

"We'll see how Darin Ruf handles first base," Amaro said, "then go from there."

Amaro was critical of Howard's performance last week. Both Amaro and Charlie Manuel have attributed his difficulties to his leg problems. But how much is age a factor?

"I mean, it could be a combination of things," Amaro said. "When you have problems and you have discomfort, it's hard to maintain your optimum level of production."

Extra bases

Washington traded Monday for noted Phillies killer Scott Hairston of the Chicago Cubs. Manager Davey Johnson expects him to start at least one, maybe two games in the series. . . . Jeremy Horst (elbow inflammation) will pitch for Lehigh Valley on Tuesday and aims to throw Thursday and Friday, too. He could be activated before the all-star break. . . . Kratz went 0 for 3 for the IronPigs on Monday. . . . Michael Stutes (biceps tendinitis) threw on flat ground Monday. "He's feeling pretty good," Amaro said.

Phillies Notes: Coming Up Lame on the Left

Ryan Howard's trips to the disabled list:

May 10-24, 2007: Left quadriceps strain.

Aug. 2-20, 2010: Left ankle sprain.

April 4-July 5, 2012: Recovering from left Achilles surgery.

Now: Torn meniscus, left knee.