The Indianapolis Colts still have nearly 1,600 unsold seats for their regular-season opener Sept. 16 at Lucas Oil Stadium, and team officials plan to announce Thursday that many of those will go on sale this week through single-game tickets.

It will mark the first time since 2003 the Colts have not sold out their home venue through season-ticket sales.

“We’re at a time with the first preseason game approaching, we felt we had to put single-game tickets on sale,” said Colts Senior Vice President Pete Ward.

The preseason home schedule kicks off Aug. 12 against the St. Louis Rams with the regular-season home opener set against the Minnesota Vikings.

Boosted by a string of nine straight playoff appearances, the star power of quarterback Peyton Manning and a brand new stadium, the Colts haven't had a problem with ticket demand in recent years. But the Colts are coming off a 2-14 season, and Manning has gone to the Denver Broncos.

In late May, Colts officials announced they had 3,000 season tickets remaining. In mid-June, the team for the first time hired an outside ticket sales firm—Indianapolis-based Get Real Sports Sales—to try to sell the remaining inventory. Fewer than half of those were sold.

Things had been so good for the Colts over the last decade, team officials essentially dismantled their sales team.

“It didn’t make sense to keep people on staff because we didn’t have anything to sell,” said Greg Hylton, Colts vice president of premium seating and ticket sales.

When ticket sales plummeted after the 2012 season, it was more efficient to bring in an outside firm than to reconstruct the sales department, Hylton said.

In June, Hylton remained confident the Colts would sell out of season tickets by early August, but a number of factors—including a sluggish economy—have hampered sales.

The 2013 season is a stunning change from the recent past.

Just five years ago, the Colts boasted a season-ticket waiting list of 30,000. Part of that list was diminished by the move from the 54,000-seat RCA Dome to the 63,000-seat Lucas Oil Stadium in 2008.

In 2010, Colts officials said they still had 16,000 people who had paid a $100 deposit to be on the waiting list.

But the season-ticket renewal rate dropped six percentage points, to 87 percent this off-season, shrinking the waiting list to 7,500. And after last year’s disappointing season, and an uncertain future featuring a new general manager, coach and quarterback, most of those left on the list took a pass on available tickets.

Some told the Colts they passed for financial reasons while others said they wanted to wait for better tickets to become available.

While the economy hasn’t helped the Colts, sports business experts say the team might have underestimated the impact of Manning’s exit.

Manning didn’t play last year due to a neck injury. And, in March, instead of paying Manning a big roster bonus written into his contract, Colts owner Jim Irsay decided to cut the all-pro quarterback, who signed a five-year deal to play in Denver. NFL marketing experts say it's likely many Colts fans are still peeved about the move and aren't willing to spend big dollars on the team.

“Peyton is unlike any athlete this market and maybe this league has seen, certainly in many, many years,” said Marc Ganis, a Chicago-based sports business consultant who counts several NFL teams as clients. “It’s an understatement to say he was much beloved in that market. There was a real connection, a kind of uncommon bond, between this guy and the people of Indianapolis. That is difficult to replace.”

But Ward says there’s reason for optimism. Colts training camp in Anderson opened Sunday to a crowd of 5,000 fans, with nearly that many showing again on Monday. Ward said 76 season tickets were sold on Tuesday alone as the “buzz is building.” Much of the training camp is open to the public, and team officials are anticipating 80,000 or more fans will descend on Anderson over the next two weeks.

“The fans are just getting to know our new players and coaches,” Ward said.

Ward added that there's a lot of excitement from fans regarding Andrew Luck, who was drafted No. 1 overall in this year’s draft and is expected to replace Manning as quarterback.

Ward is still optimistic the Colts will sell out all of their regular-season games, keeping alive a streak that dates back more than 10 years.

“So far, the camp has been packed, and that’s what we expect for our [home] games,” Ward said. “The goal is to sell them all out, and that’s what we expect.”