There's a perception around the NFL that the undefeated Pittsburgh Steelers have yet to play a strong opponent. The defense has looked great, but is it great?

Consider their opponents over the season's first five weeks: Pittsburgh's first four wins (with a bye mixed in) came at the expense of the Giants, Broncos, Eagles and Texans—teams whose current rankings in points per game are 31st, 29th, 22nd and 21st, respectively.

They dominated a better offense this past Sunday: the Browns, 12th in points per game and first in rushing yards through Week 6. The Steelers forced Cleveland's quarterbacks into a 15-of-28, 145-yard day. They held the Browns to 75 yards rushing, 1-of-12 on third down and 0-of-3 on fourth down. It was a 38-7 victory.

But more evidence is needed, particularly because the Steelers lost their defensive play-caller, Devin Bush (ACL), in the Cleveland game. The proof can come in Sunday's matchup against the 5-0 Titans in what might be the game of the week. It will feature two of the league's three undefeated teams and a chance to judge whether this Steelers defense is the real deal or has just stifled lesser opponents.

Tennessee's scoring average (32.8) is second-best in the league, while Pittsburgh has allowed 18.8 points per game. Ryan Tannehill has been sacked five times in five games, while the Steelers are averaging 4.8 sacks per game. They sacked Broncos quarterbacks seven times, Carson Wentz five times and Deshaun Watson five times.

"Best front seven in football," one veteran scout offered. His team hasn't and won't face Pittsburgh in the regular season, but he has been so impressed with the Steelers play that he pivoted a conversation about his own team to Pittsburgh's defense.

That front seven includes two All-Pros, tackle Cam Heyward and linebacker T.J. Watt. It has nose tackle Tyson Alualu, defensive end Stephon Tuitt and linebackers Bud Dupree and Vince Williams. And then there's Bush's replacement, Robert Spillane, who'll have to epitomize the defense's strong depth if the unit is to hold onto that scout's assessment.