The 36-year-old Harris starred with the Packers from 2003 through 2009. He has been playing in nickel and dime packages but was with the first string on Monday when the winless Rams (0-4) returned from their bye.
''I feel awesome, life is great,'' Harris said. ''We're all living the dream, we've just got to get some wins.''
The Rams also will give cornerback Rod Hood, signed in late September and coming off reconstructive knee surgery in June, his first playing time since 2009. Hood stayed in St. Louis during the bye, and watched film with a defensive assistant and by himself.
''I just tried to get myself in the playbook as much as possible,'' Hood said. ''It's unfortunate what happened, but it's time for some guys to step up.''
Coach Steve Spagnuolo has been reluctant to give Harris too much playing time. He noted the team is running out of options after losing Bradley Fletcher (knee) and Ron Bartell (neck) for the year.
Seven cornerbacks have been injured since training camp, including Jerome Murphy (broken ankle), a third-round pick in 2010. The Rams could get Marquis Johnson (knee) back from the PUP list next week.
''We want to take a look at everybody we have,'' Spagnuolo said. ''Rod and Al are going to have to play some football, otherwise we don't have enough.''
Harris would start opposite Justin King, who inherited Bartell's starting spot. King has been burned for four touchdown passes the last two games.
The patched-together unit will try to contain the Packers' Aaron Rodgers, who threw for 396 yards and two touchdowns Sunday night in a 25-14 victory over Atlanta. Rodgers spread the ball to a dozen receivers.
''They're scary good,'' Spagnuolo said. ''But we've got players out here, too. We've got to go play the game. We've got to find a way to slow them down and see if we can't have one more point than they do.''
Spagnuolo pointed out when he was an Eagles assistant, Philadelphia made the playoffs in 2003 despite a rash of injuries to Brian Dawkins, Bobby Taylor and Troy Vincent. Hood and Rams safety Quintin Mikell were rookies on that team.
Harris, a two-time Pro Bowler, scoffed at the notion he's too broken-down to be a major contributor.
''I really don't know where that came from,'' Harris said with a chuckle. ''But that's definitely not the case. I'm ready to contribute where they need me.''
A Green Bay homecoming means nothing to Harris, though.
''I spent eight years there and had a great time, played a lot of ball, played a lot of snaps,'' Harris said. ''But today is today and we've got to prepare like that.
''It's a football game, it's a football game. That's how you've got to treat it and that's how I'm going to approach it.''
Spagnuolo said it appears Fletcher injured his left knee when he stepped on Mike Sims-Walker's foot after batting down a pass during a one-on-one drill last week. Fletcher missed most of his rookie season in 2009, needing two operations to repair damage to his right knee.
Offensive guard Jacob Bell (hamstring), tight end Mike Hoomanawanui (concussion) and defensive lineman Darell Scott (thumb) did not practice. Bell stayed in St. Louis for rehab and Hoomanawanui ran Monday, so both could return Wednesday.
The Rams also addressed two positions hit by injuries, signing cornerback Brian Jackson and wide receiver Nick Miller, and both were on the field.
Miller played 11 games for the Raiders the last two seasons. He was used most heavily on punt returns, and totaled three catches for 40 yards.
Jackson played 12 games for the Giants last year and was waived with a hip injury earlier this season.