While the Bears' offense is concerned primarily with tweaking and fine-tuning a machine that ran smoothly last season, the defense is working on a major overhaul.
That process continues Thursday at Soldier Field against the Jacksonville Jaguars (7 p.m., ABC) in preseason game No. 2, in which some starters are expected to play into the second quarter.
Contact information ( * required )
Bears defensive coordinator Mel Tucker won't game plan for a Jaguars offense that was arguably the NFL's worst last season. Thursday night, he said, is more about how Bears defenders execute their assignments, and certain players will be more closely scrutinized with the start of the regular season just three weeks away.
"It's about us right now," Tucker said. "Just cleaning up some of the things from the previous game. We had way too many mental errors. We had too many penalties (7)."
The first roster reduction, from 90 to 75, isn't until Aug. 26. Four days later teams must be at the final 53, but coach Marc Trestman said he's not concerned yet with paring the roster.
"I don't want to stand here and start figuring out what our 53 and 46 (eligible on game day) are," Trestman said before his team broke camp in Bourbonnais. "I'm just going the wait-and-see route."
The performances of linebackers Shea McClellin and Jon Bostic, defensive ends Trevor Scott and Austen Lane and rookie cornerback Kyle Fuller all bear watching against Jacksonville.
McClellin is in the process of converting from defensive end to strongside linebacker. Reviews by coaches mostly have been underwhelming, although no one expected McClellin's transformation to be completed overnight.
"For the most part he blended in," linebackers coach Reggie Herring said of McClellin's first preseason game, in which he had 2 tackles. "He's got to put himself in better position to make some tackles. But those are things that come with (experience)."
At Boise State, McClellin played both positions well enough for the Bears to make him their first-round draft pick (19th overall) in 2012. Herring said McClellin has flashed enough for coaches to be optimistic.
"Shea's shown enough out there that I believe he has linebacker instincts," Herring said. "I think he's going to be fine. There are times out there when he moves better than all of them -- change of direction, instincts, breaking on the ball.
"Y'all be patient. We'll know after the first (regular-season) game, second game where we're at with him. But right now he's on schedule. He needs more plays, more games, and I really believe that he'll come and he'll be a solid player for us."
McClellin has to do a better job of carrying over production in practices to games for that to happen.
"He's working at it," defensive coordinator Mel Tucker said. "We see him do it in practice. He looks very instinctive in practice, and we saw him make sudden movements and quick decisions in the game."
Jon Bostic, last year's second-round pick, was unimpressive in the opener and needs to play better. He's getting snaps at middle linebacker, but veteran D.J. Williams has a firm grasp on that job.
Bostic had been getting extensive practice snaps with the first team in nickel situations, but he was abruptly pulled from that spot early in last week's game. He's also in the mix at strongside linebacker.
Khaseem Greene, last year's fourth-round pick, is battling for playing time.
"Right now, Shea and (Bostic) are still works in progress," Herring said. "Khaseem is doing some really good things to show that he is committed to being a good football player, and he's showing improvement each day."
The same could be said of Fuller, which is more impressive considering he's going against Pro Bowl receivers Alshon Jeffery and Brandon Marshall every day in practice and playing cornerback in the No. 1 nickel defense in games.
"Nothing ever seems to faze him," said defensive backs coach Jon Hoke of the first-round pick. "He's just a guy where there's never really any high, never really any low. He's the same every day, and that's a good thing. And he loves to play football."
Tucker was impressed by Fuller's first game as a pro, which included 3 solo tackles.
"He tackled well, which is important," Tucker said. "We want our corners to be high-percentage tacklers and willing tacklers, and he did that. It was not too big for him."
Lane spent three years (2010-12) as a backup with the Jaguars when Tucker was the defensive coordinator in Jacksonville. Scott, who led Bears defensive linemen with 3 solo tackles last week, has six previous seasons in the league. Both are competing with David Bass to fill the one or two backup spots behind Jared Allen, Lamarr Houston and Willie Young.
"Trevor showed good anchor vs. the run and was active vs. the pass," Tucker said. "That was consistent with what he's done in practice.
"Austen is a very physical player. He's developed as a rusher since I had exposure to him in Jacksonville. It was very positive for both of those guys."
• Follow Bob's Bears and NFL reports on Twitter@BobLeGere.