<P>Veteran tight end Greg Clark will undergo surgery on his troublesome right hamstring on Tuesday, his third procedure to repair the injury since March. </P>

"Obviously, it's a different injury altogether," 49ers coach Steve Mariucci said Monday afternoon. "And it is a new sort of surgery and injury on the hamstring. Hamstring surgery is very rare, as far as I know. We're just trying to get it fixed, get it right."

Clark hasn't been right in quite some time. A regular starter since 1998 who had a career-high 38 receptions last year, Clark didn't participate in training camp practices before undergoing surgery Aug. 14 to remove a portion of his medial hamstring tendon. It was originally projected Clark would be able to return to action two to four weeks after that surgery. But he has continued to have complications, and this follow-up surgery is intended to clean out scar tissue. The timetable for Clark's return to action remains uncertain.

"I don't know the answer to that," Mariucci said. "I think after they do the surgery, then they'll give us a better idea. We'll know more after that. It's a setback in terms of weeks being out. We were hoping to have him right about now, or soon anyway. But he simply hasn't been able to practice."

Justin Swift started the Sept. 9 season opener against Atlanta in Clark's place, and rookie Eric Johnson was the starting tight end on Sunday against the St. Louis Rams. Johnson is second on the team with seven receptions, and Swift has four catches.

INJURY UPDATE --- Starting left tackle Derrick Deese had an MRI exam on his right calf late Monday and will be listed as questionable for next Monday's game against the New York Jets.

 "He seems to be optimistic he'll be able to play," said Mariucci, who said Deese will wear a boot on his leg this week to rest the injury.

--- Starting defensive tackle Bryant Young also will be wearing a boot this week on his left ankle, which he sprained late in the fourth quarter against the Rams. He's also listed as questionable for the Jets game.

--- Tailback Garrison Hearst suffered a concussion in the second quarter Sunday and was held out the rest of the game. But Mariucci said he was feeling fine Monday and should be ready to start at New York.

--- Linebacker Derek Smith, who had 13 tackles Sunday, suffered a Grade 2 separation of his left shoulder. He's listed as probable, as is versatile running back Terry Jackson, who has a sprained foot.

--- Young linebackers Julian Peterson and Jamie Winborn, neither of whom has played so far this season, both will practice without limitations this week and will make their season debuts against the Jets.

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