GREEN BAY, Wis. – The Chicago Bears' defense is still playing catch-up.

There are multiple reasons why the Bears collapsed and lost 24-23 to the Green Bay Packers on Sunday night – the fact that Aaron Rodgers is one of the greatest quarterbacks of all time tops the list – but you have to wonder whether the defense would’ve fallen apart in the fourth quarter had Khalil Mack and Roquan Smith been at full strength.

Mack and Smith’s conditioning can’t directly be blamed for Geronimo Allison burning Kyle Fuller for a 39-yard touchdown. Or for Fuller dropping a critical interception in the fourth quarter. Or for Randall Cobb shredding the Bears’ defense for a 75-yard game-winning touchdown.

But the reality for the Bears is that Vic Fangio’s defense will reach its full potential only when Chicago’s top two offseason defensive pickups are in good enough shape to play most of the snaps at their respective positions.

Mack looked superhuman in Sunday’s first half. With only a handful of Bears practices under his belt, Mack dominated the Packers, taking the ball away from Green Bay backup quarterback DeShone Kizer on a strip sack/fumble recovery, and later intercepting a Kizer pass and returning it 27 yards for a touchdown.

“In the first half, Mack was pretty much what we thought, but we didn’t know it for sure,” Bears coach Matt Nagy said. “Like I told you, we didn’t know how many reps he would get or where he was at conditioning-wise. He was doing well, feeling well and making big-time plays.”

However, Mack wasn’t nearly as effective in the second half, partly because Rodgers was getting the ball out so quickly, but also because the former NFL Defensive Player of the Year held out all of training camp and the preseason before the Raiders traded him to Chicago. That Mack played so well is a testament to his All-Pro talent, but there’s no way he entered Week 1 in his best possible football shape.

Mack said after the loss that he “loves the game. When you love it, it’s easy [to come back after missing all of the preseason].”

Mack’s enthusiasm and passion is obvious, but so too is the fact he needs more time to be truly comfortable in Fangio’s scheme.

The same logic applies to Smith, who came off the bench – Nick Kwiatkoski started at inside linebacker -- only after Danny Trevathan temporarily left the game due to injury. Almost on cue, Smith picked up a sack on his first career NFL snap, but the rest of his rookie debut was rather nondescript.

The Bears drafted Smith eighth overall to be an every-down linebacker, which he undoubtedly will be in the near future. Smith never came off the field at Georgia, and the same role awaits him in Chicago. But Smith missed most of camp with a contract dispute and a hamstring injury. And Smith doesn’t have the luxury of NFL experience, like Mack, nor does he play a position that requires him to almost exclusively rush the passer, again, like Mack.

The Bears were close to a top-10 defense last year, but they had two major holes: inside linebacker and pass-rusher. The front office filled both of those needs in the offseason, but with strings attached.

Among the many things learned in Week 1: The Bears' offense isn’t the only phase that should be considered a work in progress.