Am I mistaken, or don't they play two completely different positions? How could Urlacher be a replacement for Harrison? He's always been a middle linebacker in the 4-3. I definitely wouldn't think he could translate to an outside linebacker in a 3-4, but could he even learn inside linebacker in a 3-4 this late in his career?
N'way could he play OLB in a 3-4. Don't think he could play the Mack, either. Timmons and his speed fits that role better. He could probably play the Buck, but he'd have to learn the position and want to take on that role.
His age is a concern, but I'm pretty sure that if healthy, he's going to be playing with a darn big chip on his shoulder. He'd definitely be a solid locker room presence as Finny mentioned, even though he threw that out there in jest.
Bottom line, this team needs players/playmakers, veteran leadership and
depth. They ain't gonna get it all from the draft, that's for sure. If they can bring in a couple vets that still have something left to prove and they're good team guys, it helps to address two of those issues.
In the end though, it'll boil down to the monies.