Incumbent starter DuJuan Harris is itching to play after missing time with a knee injury.
Rookie Eddie Lacy has Green Bay fans buzzing after showing off his spin move and power-running style in his preseason debut last week against the Rams.
The running backs will be in the spotlight in the Packers' third preseason game Friday night at home against the Seahawks, what McCarthy dubs the "dress rehearsal" for when the real games start in a few weeks.
"It's time for individuals to step up and claim some responsibility," said about the team-wide competition for playing time. "You want to see this thing start to define itself. This is not a fun time of year. Who's doing what? You're running out of reps because we're halfway there."
So Harris, who is supposed to play Friday, is returning just in time to try to make an impression — just as Lacy did last week against St. Louis.
Dreadlocks flowing out from under his yellow Packers helmet, the second-round pick from Alabama gained 40 yards on eight carries in the first half and added an 11-yard reception.
It was limited time in one preseason game, but Lacy made a successful debut nonetheless.
"I'd like to see Eddie clean up the details of his work. He did some really good things in St. Louis. He had two excellent chips in pass protection, ran the ball very effectively," McCarthy said after practice Monday. "He's got a lot more in front of him as far as details of our offense."
Lacy ran for more than 1,300 yards and 17 touchdowns on 204 carries last season for top-ranked Alabama. He's handled himself well on and off the field in the pros, said offensive tackle Marshall Newhouse, a four-year veteran.
"Obviously on the field, he's a finisher. He leans forward, gets extra yards," Newhouse said. "He's come into work in the weight room, on the field and done all the right things."
Lacy has the momentum, but McCarthy is still sticking with Harris for first-team reps. Lacy and fourth-round pick Johnathan Franklin followed Harris in the running back rotation Monday. Veterans James Starks and Alex Green are on the roster, too.
Harris has earned his shot by virtue of cementing his role as a starter by the end of 2012 after joining the team from the practice squad in October.
He rushed for 157 yards and two touchdowns in four games, and caught a team-high five passes in the NFC wild-card win over Minnesota.
"Of course, I'm a lot fresher than some guys, but I'm back," Harris said. "It's real important to get those hits out of the way, and pretty much get a groove on things."
While the passing game under quarterback Aaron Rodgers remains the headlining act, any improvements in the running game should help the rest of the offense.
Last season, the Packers ranked 11th in the NFC at 106.4 yards rushing per game. They haven't had a single-game, 100-yard rusher in almost three years.
Against the Rams, the Packers provided a glimpse of how diverse the offense could be if opposing defenses need to worry about the running game.
"Not only for me, but for all the guys period," Finley said when asked how an improved running game could help the offense. "We've got Lacy back there, we've got Harris. It's going to be nice to see how (defenses) are going to play us this year."
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