The Illini defense, one of the teams strengths through a 5-3 start, suddenly found itself allowing almost 40 points in each of the final four games.
Illinois, though, has another game to play before Zook and his staff begin tweaking. The Illini face Baylor on Dec. 29 in the Texas Bowl -- and the Bears are rolling up almost 480 yards and 32.6 points a game.
Sound like anybody Illinois (6-6, 4-4 Big Ten) faced this season?
"I think if you look at formation-wise, you'd probably think Missouri," Zook said of the Tigers, a team Illinois held to 23 points in a season-opening loss.
"If you look at the quarterback, you probably think of Michigan or Ohio State; he's kind of a combination of the two," Zook said, referring to the Wolverines' Denard Robinson and the Buckeyes' Terrelle Pryor.
Zook was talking about Baylor quarterback Robert Griffin and his "track team" of receivers. Griffin has thrown for 3,195 yards and 21 touchdowns and he's run for 591 yards with eight more TDs in leading Baylor to a 7-5 record. It's the type of quarterback Illinois handled early on in the season, but could not deal with down the stretch.
Pryor was limited to 76 yards passing and 104 rushing in leading then No. 2 Ohio State to a 24-13 win over the Illini early in the season. Illinois led 14-10 heading into the fourth quarter, so new defensive coordinator Vic Koenning figured the team was heading in the right direction.
Five weeks later, Michigan piled up 676 yards, with Robinson and backup Tate Forcier combining for 419 yards passing and five touchdowns.
Zook said the game took its toll on the defense.
"We played Ohio State, Penn State, Michigan State, Purdue and Indiana and we played really great defense," he said. "Then we run up against a team that probably played their best game maybe in the last three years, offensively, at Michigan and never really recovered from it in the Minnesota game (a 38-34 loss)."
Safety Tavon Wilson said Griffin is probably as good as any quarterback the Illini have seen this year.
"We've faced some pretty good offenses, but this quarterback, he can do a lot with his feet and his arm," the junior said.
Wilson can't explain Illinois' late-season struggles against Michigan and Minnesota.
"Against teams that were versatile, we just feel like we didn't do that very well, for whatever reason," he said.
Zook's job will be to figure it out and make the necessary changes. For now, rest was probably a good thing. The Illini haven't played since their loss Fresno State on Dec. 3. They were given a few days off and look like they're ready to go.
"I think just watching them practice and the way they're attacking and the things we're stressing and going back to, I'm looking forward to this game as much as everyone else is," Zook said.