PITTSBURGH -- The Steelers officially placed outside linebacker T.J. Watt on injured reserve, ruling him out for at least four games.

Watt, the reigning NFL Defensive Player of the Year, tore his pectoral muscle in the final seconds of regulation in the Steelers' Week 1 win against the Cincinnati Bengals. But Watt won't require surgery, a source told ESPN's Adam Schefter, and he could return in about six weeks.

"I can definitively say that T.J. won't play this week, but I won't make any commitments beyond that," coach Mike Tomlin said Tuesday. "We're encouraged and we'll just continue to look at the situation and gain opinions and do what's appropriate."

In a corresponding move, the Steelers signed outside linebacker David Anenih to the 53-man roster off the Tennessee Titans practice squad. Anenih signed as an undrafted free agent in May and finished the preseason with three sacks. The Steelers also signed former Washington Commanders linebacker Ryan Anderson, an Alabama product, to the practice squad.

Without Watt, the Steelers will lean heavily on third-year outside linebacker Alex Highsmith, who sacked Joe Burrow three times on Sunday. He also recorded four quarterback hits.

"I don't know that any of us are shocked by the performance," Tomlin said of Highsmith. "As a matter of fact, I think most of us expect it to continue. You're not going to get three sacks each and every week, but he's a good player and he's a man to be reckoned with for sure."

The Steelers will also use outside linebacker Malik Reed, who they acquired in a trade before the season, and Jamir Jones, who was recently picked up off waivers, to help fill Watt's extended absence.