The Steelers got by for one night, barely, without a running game. What could be more difficult: getting by the first half of the season without All-Pro safety Troy Polamalu.

Polamalu, a game-changing talent who might be one of the NFL's most irreplaceable players, sprained the medial collateral ligament in his left knee during a blocked field goal in Pittsburgh's 13-10 overtime victory over Tennessee on Thursday night.

Even before Polamalu underwent more tests Friday to further evaluate the injury _ a sprain is a tear in the ligament _ Steelers coach Mike Tomlin estimated he would be out from three to six weeks. Players have returned from such an injury within a month, but not often.

After winning their seventh consecutive opener, the players were given the weekend off and they won't practice again until Monday. The team may not offer an injury update on Polamalu until then.

As the Steelers learned immediately after the Titans' Alge Crumpler fell on Polamalu's knee as it was bent at an awkward angle, not having Polamalu for three to six plays is difficult enough.

"Troy is one in a million," defensive coordinator Dick LeBeau said. "We'll play well without him, but you don't replace Troy."

While linebacker James Harrison was the Defensive Player of the Year last season, Polamalu gives the Steelers' top-ranked defense its uniqueness. He lines up everywhere from the line of scrimmage to the deep secondary, blitzing on one play and dropping into coverage the next, and is the key element of the Steelers' varied and numerous zone blitzes.

Polamalu was the best player on the field before he was hurt, making six tackles, a one-handed interception of a Kerry Collins pass _ with his left hand _ and dropping Chris Johnson for a 2-yard loss on a third-down blitz, all in less than a half. Polamalu's repeated big plays helped keep the game close even as the Steelers were rushing for only 36 yards on 23 carries.

"He definitely was everywhere," linebacker LaMarr Woodley said.

To the Steelers, losing Polamalu on defense for an extended period might be the equivalent of the Patriots losing Tom Brady on offense. Brady injured a knee in the opener last season and the Patriots didn't make the playoffs despite going 11-5.

"It's definitely going to be difficult without him, he's one of the best players in the league," safety Ryan Clark said. "Preferably, we get him back as soon as possible."

Immediately after Polamalu was hurt, Collins drove the Titans three plays for their only touchdown, hitting rookie Kenny Britt for 57 yards with no defender in sight, and an equally wide-open Justin Gage on a 14-yard touchdown play.

Without Polamalu, it was obvious there was confusion in the secondary. The Steelers had only 10 defenders on the field on one play _ and, without their playmaker, it may have seemed like they had fewer than that.

"At first, I don't think we did too well," Harrison said. "The communication wasn't there. We came back in and did some adjustments and went back out and did all right."

With Tyrone Carter mostly filling in for Polamalu, the Steelers held Tennessee to only a field goal during the second half and overtime. However, a defense that held opponents to a league-leading average of 156.9 yards passing last season allowed 234 to the Titans, even though Collins had only four games of 200 yards or more last season.

"You've got to look at it as you've got to do your job. I think we kind of got out of that mode once he went down," Clark said. "Coach LeBeau comes in (at halftime) and tells us, `Troy's not coming back out, but we've got what it takes to win this game.' We were able to do it."

The Steelers (1-0) won't have Polamalu for two important upcoming games, Sept. 20 at Chicago and Oct. 4 at home against San Diego, which Pittsburgh beat in the playoffs last season.

"The defense changes a lot (without Polamalu)," Woodley said. "Troy is a real big part of our defense. But you can't worry about injured guys _ the next guy that goes in there is going to be representing the Pittsburgh Steelers, and we continue to move on. Troy's not here next week, (so) we've just got to count on the guy and believe in the guy who's coming in for him."