Urlacher was injured on the second play of Oakland's opening 81-yard touchdown drive.
"On like the third play, I was running and I felt like a cramp," Urlacher told the Tribune. "Then, on the next play, I couldn't put pressure on it. Then I was out. ... I had a similar-type injury (in 2007) on a Tuesday, and I ended up playing that Thursday (at Washington). I'm not going to speculate on how long, but I'll be fine."
In 2004, Urlacher missed seven games with a lower left leg injury that required surgery to relieve the pressure of internal bleeding.
The Associated Press contributed to this report.