Lewan tweeted that an MRI taken Monday confirmed the ACL tear, adding: "Don't feel sorry for me, I'm going to handle this rehab like a Pro and come back better than ever."
Lewan, who was placed on injured reserve on Monday, sustained 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.
"Disappointed for Taylor," Titans coach Mike Vrabel said Monday. "He worked extremely hard. We're confident that he'll be back. But that's kinda what we've done; that's all we really know. That's how we try to operate is that we'll get the next guy in there, and we'll see what's best and try to move forward with the next guy in there."
Lewan played 226 offensive snaps for the Titans this season. He played 40 snaps before suffering the injury Sunday. The Titans gained an average of 3.9 yards per rushing attempt when running to Lewan's side.
Veteran offensive tackle Ty Sambrailo came in when Lewan was injured. Sambrailo gave up a strip sack to J.J. Watt on his first snap but settled in as the game went on, and he'll likely be the left tackle going forward.
Vrabel said Sambrailo's versatility and experience is an asset. Sambrailo has played left and right tackle along with guard in past seasons with the Broncos and Falcons.
"I am confident that we will have five guys that will be comfortable where they're at and try to learn the game plan to be ready," Vrabel said. "We'll have some guys that are available in a backup role and have some versatility to play more than one position."
Now in his seventh season, the 29-year-old Lewan has been selected to three Pro Bowls.
"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!"
I wanted to let everyone know I got an MRI today. I Tore my ACL yesterday against Houston. Don't feel sorry for me, I'm going to handle this rehab like a Pro and come back better than ever. Thank you @Titans fans for your support. #NoBadDays— Taylor Lewan (@TaylorLewan77) October 19, 2020
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.
In another move on Monday, the Titans activated wide receiver Corey Davis from the reserve/COVID-19 list.
ESPN's Turron Davenport contributed to this report.