INDIANAPOLIS -- A rookie quarterback and four backups on the offensive line didn't prevent the Colts from torching the Vikings defense in clutch situations at the end of each half in the Colts' 23-20 victory at Lucas Oil Stadium on Sunday.

On their final drives in the second and fourth quarters, the Colts moved with ease, going 64 yards in 1 minute, 4 seconds for a touchdown before halftime and 45 yards in 23 seconds to set up the game-winning 53-yard field goal by Adam Vinatieri with eight seconds left.

The late-game collapse was particularly concerning for a team that believes it significantly upgraded its secondary this season with the return of Chris Cook and Antoine Winfield and the addition of rookies Harrison Smith and Josh Robinson.

"That's embarrassing," defensive end Jared Allen said of the Colts' final drive. "I don't know. You know, it's just, I got nothing to say. ... We got to find a way to get the ball down."

By the middle of the second quarter, Colts rookie quarterback Andrew Luck was working behind four backups when center Samson Satele went out because of a knee injury. Left guard Joe Reitz (knee) and right tackle Winston Justice (concussion) were inactive, replaced by former Vikings practice squad player Seth Olsen and Jeff Linkenbach, respectively. Right guard Mike McGlynn moved to center and newly signed Trai Essex to right guard when Satele left the game.

Yet the Vikings managed only two sacks and no takeaways as Luck posted a 107.5 passer rating in his second NFL game.

"[Luck] is elusive and has good pocket presence," Allen said. "Most quarterbacks we face, we'd have had six or seven sacks today."

In his last possession of each half, Luck completed six of eight passes (including one spike to stop the clock) for 93 yards to four different receivers. Reggie Wayne caught three of them for 59 yards, including a 30-yard touchdown on a perfectly thrown pass to beat the Vikings' two-deep coverage with seven seconds left in the first half.

Lined up in their nickel defense, the Vikings had middle linebacker Erin Henderson trying to run step for step with Wayne. When the ball is thrown, the safeties, who are playing deep half, are supposed to provide help over the middle.

Wayne got behind Henderson and caught the ball before either Smith or Mistral Raymond could close in.

"It's coverage, it's rush, it's everything working together," Raymond said. "And we didn't execute any of it."

The Vikings came back from that touchdown to tie the score late. But a soft zone coverage allowed Luck to complete back-to-back 20-yard passes with little resistance.

"He got the ball out pretty fast all day," defensive tackle Kevin Williams said. "And guys were just open. It's just something we have to fix."

If it's not fixed, the Vikings are in trouble. Jacksonville's Blaine Gabbert and Luck -- two youngsters with a combined 15 starts before they played the Vikings -- just spent the past two weeks completing a combined 61.4 percent of their passes for 484 yards, four touchdowns, no interceptions and a 101.1 passer rating against the Vikings.