"I spoke to him this morning, I excused him. It's not injury-related. He's taking care of some personal issues," head coach Doug Marrone said. "I'm going to talk to him this afternoon. ... I don't know much about the situation."
Lawson, 29, has started eight games this season for the Bills, recording 43 tackles, one sack and one forced fumble.
Meanwhile, wide receiver Robert Woods (ankle) is listed as doubtful but will travel to Pittsburgh with the team. He did not practice again Friday.
Fellow rookie receiver Marquise Goodwin (hamstring) is probable and will play Sunday, per Marrone.
The full Bills injury report, including player participation in Friday's practice:
WR Robert Woods (ankle; did not participate)
LB Manny Lawson (personal; did not participate)
QB EJ Manuel (knee; full participation)
QB Thad Lewis (ribs; full participation)
DT Kyle Williams (Achilles'; full participation)
WR Marquise Goodwin (hamstring; full participation)