Lewan tweeted that and MRI taken Monday confirmed the ACL tear and added: "Don't feel sorry for me, I'm going to handle this rehab like a Pro and come back better than ever."
Lewan suffered the injury in the third quarter of Sunday's overtime victory over the Houston Texans. Tennessee rallied without the three-time Pro Bowler to improve to 5-0.
"I can't wait to watch my guys crush it," Lewan said Monday in a follow-up tweet. "I'm going to be the biggest titans fan each and every Sunday. We are doing something special here and this small set back doesn't change a thing!"
Now in his seventh season, the 29-year-old Lewan has been selected to three Pro Bowls.
Also, Titans tight end Jonnu Smith is dealing with a "minor" ankle sprain and is expected to be considered questionable for Sunday's game against the Steelers, a source told ESPN's Adam Schefter.