Peterson tears Achilles tendon

A 49ers season already on the fast downward spiral took another wicked spin in the wrong direction Sunday against the Arizona Cardinals when All-Pro linebacker Julian Peterson suffered a torn left Achilles tendon near the end of the first quarter. Four plays later, backup fullback Jasen Isom tore his left Achilles blocking on a kickoff return.

Peterson - the fifth-year veteran universally considered the Niners' best player - will miss the remainder of the season. The same holds true for Isom, who started one game at fullback this year and was a key player on San Francisco's special teams.

"I have to get surgery," Peterson said after the game. "I'm getting an MRI (exam) tomorrow, so we'll see what happens. All I can do is wait for next year."

An Achilles' tear usually requires a six-month recovery period before a player is ready to return to football-like activity.

It usually takes four months after such an injury before a player can even begin running again, and two to three months before he even can walk without the help of crutches or a protective boot.

"It's just two devastating injuries," 49ers coach Dennis Erickson said. "That's just a crime to have something like that happen to those guys. (Julian)'s the leader of our defense. When you lose a guy like that, it is hard. We have to make up for him being gone."

Peterson was hurt on a first-down play in a 0-0 game with 90 seconds remaining in the first quarter.

Peterson - whom the Niners move all over the field in their defensive schemes - was coming off the left edge on an Arizona passing play when he was chop-blocked by offensive tackle L.J. Shelton.

Peterson attempted to dance out of the way of the block, but Shelton caught him from behind with a leg whip while Peterson was in the air. He then landed awkwardly on his foot. Peterson not fall to the ground before limping off the field.

"I'm disappointed I went out on a play like that," Peterson said. "The guy tried to cut-block me and couldn't take me down, so he leg-whipped me and took out my Achilles."

The Niners already were without four regular defensive starters entering Sunday's game. Right cornerback Mike Rumph is out of the season with a broken right forearm, linebacker Derek Smith is out until at least the end of this month with torn ankle ligaments, right defensive end Andre Carter is out until at least November after having surgery on his back last week and left end John Engelberger didn't start Sunday because of an abdominal injury.

Whiting suffered a high ankle sprain Sunday and is listed as out for next week's game against the New York Jets. Safety Dwaine Carpenter, linebacker Brandon Moore (who replaced Peterson), linebacker Saleem Rasheed and cornerback Ahmed Plummer all suffered minor injuries in the game and are probable next week. Cornerback Jimmy Williams suffered a concussion and is listed as questionable.

Peterson was on his way to another impact performance before he was hurt. He already had two tackles and had batted away a pass on a play that exhibited his tremendous skills.

Arizona quarterback Josh McCown rolled to his right on a misdirection play, and Peterson rolled with him while also staying in the way of the Arizona receiver who had drifted into the flat - the intended receiver on the play. At the last second, Peterson left the receiver and charged for McCown, knocking the pass into the air as the quarterback released it. Peterson almost turned around and intercepted the pass, too, before it fell to the ground.

But Niners fans won't be seeing any more of that this season. Six minutes later, Peterson went down for the season in the latest bit of bad luck and misfortune to hit the team.

At the time he was hurt, Peterson ranked second on the team with 22 tackles, was first with 2.5 sacks and had two passes defensed and one forced fumble.

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