Culpepper threw for a touchdown and Stafford passed for 160 yards to help Detroit beat the Indianapolis Colts 18-17 in an exhibition game.
Three-time MVP Manning has said Stafford -- the No. 1 pick in the NFL Draft -- would be best served to play right away as he did, but left Ford Field impressed with Detroit's top two QBs.
"I thought both of them looked good," Manning said. "It seems like the race is going to go down to the wire."
Lions coach Jim Schwartz chose to begin the first and third preseason games with Culpepper under center, but wouldn't discuss plans for the exhibition finale Thursday night at Buffalo.
Schwartz did say, though, he is planning to use his top players more than most coaches do in the fourth exhibition.
"We're still in evaluation mode," Schwartz said. "We're going to approach it the same way we did these other three games."
Culpepper completed seven of 12 passes for 67 yards, deftly avoiding the rush to make some throws and showing enough poise to connect with his third option on the TD -- Bryant Johnson for a 3-yard score.
"I feel great," said Culpepper, who was a Pro Bowl-caliber QB in Minnesota before a knee injury in 2005. "I honestly feel like I did before I got hurt."
Stafford was 13 of 19 and threw an interception, which his coach blamed on Johnson for not being more aggressive.
"I feel like I've played well," Stafford said. "I don't know what they're going to do. It's not up to me."
While Detroit's QB situation is murky, Manning is still in charge for the Colts, just as he has been since they drafted him first overall in 1998.
The reigning MVP was 12 of 15 for 123 yards, including a 6-yard TD to Dallas Clark, who had six receptions for 63 yards.
"Both drives at the beginning of both halves were quality," Manning said. "On the two drives where we did not get points it was more us than them. It was us self-destructing."
Indianapolis (1-2) put several banged-up players on the inactive list, but coach Jim Caldwell wasn't in the mood to make excuses for the Lions' 412 yards of offense.
"Regardless of who goes in there and plays for us, we expect them to play as well as anyone else, whether he's a starter or not," Caldwell said. "For the most part, we just didn't get it done."
"In both cases the guys haven't had an opportunity to play much at all this preseason," Caldwell said. "So, to get them out there was very important."
- Colts DE Rudolph Hardie (knee), TE Tom Santi (ankle) and Lions DE Jason Hunter (ribs) were injured during the game.
- Detroit's Kevin Smith had 50 yards on eight carries, including a 24-yard run.
- Joseph Addai ran for 22 yards on seven carries for the Colts.
- Calvin Johnson, who missed the first two preseason games with a sprained thumb, had four catches for 67 yards and Bryant Johnson played for the first time since getting in an accident with a golf cart.