Ravens veteran center Matt Birk has been sidelined for three weeks following left knee surgery. Mike Duffy of BaltimoreRavens.com reports that Birk “remains on track to start Week 1 against the Pittsburgh Steelers.”

“He’s progressing,” Harbaugh said Sunday. “It was supposed to be 3-4 weeks before he’d be moving around pretty well, and now he’s starting to walk a little bit. I’m anxious. I want to see him out here sooner rather than later, but it’s not something that we really control.”

Duffy says that Birk has been at the Ravens training facility walking without a “noticeable limp.”

On Sunday, the Ravens added Jason Murphy to build up their depth at center. Bryan Mattison has been filling during Birk’s absence.