Leftwich and Harrington split the number of practice snaps taken by the No. 1 offense on Thursday.
Petrino wants to see how well Leftwich moves on his tender ankle on Friday before deciding which quarterback will start. The first-year coach was pleased that Leftwich arrived at team headquarters showing no stiffness or swelling after a hard practice the day before.
"We just need to make the right decision," Petrino said. "We'll have to see how it reacts and how he does."
Petrino hasn't reconsidered his move to name Leftwich the starter and Harrington the backup after Atlanta began the season 1-5.
After practice on Thursday, Petrino indicated that Leftwich this week has occasionally used an inaccurate cadence or made a poorly timed pass.
"He missed a couple of throws that we'd like to hit," Petrino said. "You know you'd have to knock some rust off. We are not going to make a decision until we have to."
Since getting released from Jacksonville and signing a two-year contract with Atlanta, Leftwich has played only twice. An ankle injury forced Leftwich to leave his only start, a 22-16 loss at New Orleans in Week 7.
Leftwich, the No. 7 overall draft pick in 2003, went 24-20 as the Jaguars' starter. Harrington, the No. 3 overall pick for Detroit in 2002, has a combined career record of 26-48 with the Lions, Miami Dolphins and Falcons.
With suspended quarterback Michael Vick facing a federal prison sentence for dogfighting, the Falcons signed Leftwich after Jacksonville cut him at the end of preseason.
Leftwich has dealt with ankle injuries throughout his career. He underwent surgery on his left ankle after the Saints loss and returned to practice last week.
Harrington signed in March to be the backup, but Vick's federal indictment made him the starter a couple of days before training camp began in July.
Harrington, who led the Falcons to victories in each of the last two games, was unhappy when reporters told him Wednesday that Petrino had reaffirmed Leftwich's status as the starter.
"But like I said a couple of weeks ago when this all started, this isn't my first rodeo," Harrington said in reference to benchings with Atlanta, Miami and Detroit. "I've been around it before, so I'm not surprised. But like I've also said, I have a right to disagree with it."
Notes: Tight end Alge Crumpler, who caught Harrington's 30-yard touchdown pass for last week's win at Carolina, skipped practice to rest his sore ankle and knee.... Receiver Joe Horn has missed both sessions this week with an injured hamstring. Signed in March as a free agent who would energize Atlanta's young receivers, Horn has just 18 catches for 165 yards to rank fifth on the team.... No. 2 running back Jerious Norwood remained on the injury report filed to the NFL, but his ankle seems to be improving. Norwood has practiced the last two days.