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During the 208 consecutive regular-season games and 19 playoff games Peyton Manning has started since he was a rookie in 1998, no player in the N.F.L. has been more intrinsic to his team’s success than he has. The Indianapolis Colts have gone to the playoffs in all but two of Manning’s 13 seasons, including the last nine in a row, and his brilliance has covered a multitude of holes on their roster. But when the season opens Sunday, and perhaps for weeks beyond, the Indianapolis Colts will probably get the answer to the dreaded question that has surrounded them for years: what would they be without Manning?

On Monday, in an unusually detailed statement, the Colts announced that Manning, who is still recovering from surgery on a bulging disk in his neck in May and has been practicing for only a week, is doubtful for Sunday’s game against the Houston Texans. More ominous is what else the team said: Manning, who began experiencing unexplained back soreness over the weekend, is being shut down in practice and the rate of improvement in his rehabilitation has slowed. Specialists are being consulted and tests are being completed, suggesting that his absence may extend further into the season.

“As of now Peyton continues to deal with a complicated neurological recovery, the end date of which is unpredictable,” the Colts’ statement said.

The Colts are typically tight-lipped about injuries, which gives added heft to this kind of candor. For them to announce early in the week that the fulcrum of their team is probably not going to play — did you hear that sigh of relief coming from Houston? — indicates how serious the situation is. According to Football Outsiders, last year no player listed as doubtful during a week actually played that week’s game, and no Colts player listed as doubtful has played since 2003. Still, in comments Monday, Coach Jim Caldwell seemed to offer the dimmest hope that Manning could play.

“It’s been an incredible feat,” Caldwell said. “He’s been an iron man, there’s no other way to put it. It’s doubtful that he plays this week, but it takes a very unusual individual to have that streak.”

For the 35-year-old Manning, of course, this must be a disturbing development. This is the most significant injury of his career and it raises questions about whether Manning’s body starting to betray him. He has famously missed only one play because of injury in his pro career, briefly leaving a game against Miami in 2001 after sustaining a broken jaw. In 2008, he had surgery in mid-July to remove an infected bursa sac in his knee and missed all of the preseason and most of training camp. He was rusty at the start of the season, but went on to win his third Most Valuable Player award.

This injury, though, is different in part because the rate of regeneration of nerves is difficult to predict. During the summer, Manning expressed frustration that he was unable to meet with his regular trainers during the lockout and he said he was taking his rehabilitation particularly slowly because he was working with unfamiliar people. But when he returned to practice last week for the first time since January, Manning was upbeat in a meeting with reporters in which he said he hoped that in consultation with doctors and coaches he would get to decide if he would play against the Texans.

That plan has now been upended and so will the balance of power in the A.F.C. if Manning misses a significant portion of the season. The Texans, a talented division rival, have been poised to reach the playoffs for several years — if only Manning and the Colts would get out of the way.

Manning has been so reliable that the Colts have had the luxury of ignoring the backup quarterback position. That is why giving the Texans a week head start to prepare for Kerry Collins, who got the Colts’ playbook little more than a week ago when he was lured out of retirement, is so unthinkable and so telling. Collins catches a break with the schedule, because he is familiar with the Texans after his years with the Tennessee Titans. In five starts for Tennessee against Houston, Collins won three games while completing 88 of 157 passes with five touchdowns and four interceptions. Collins said that when he agreed to join the Colts, he was not sure he would ever have to play.

“I thought the first week might be a possibility with Peyton’s status so uncertain,” Collins said Monday. “There’s no way I can replace someone like Peyton and what he means for this team and this franchise. But I will bust my butt to get ready and I hope the guys see that.”

Clearly the Colts’ playbook will be drastically scaled back for however long Collins plays. Collins is a smart player who knows how to prepare himself, but the Colts are ill-equipped to change their fundamental nature quickly. Their running game, with Joseph Addai and Donald Brown (neither of whom reached 500 yards last year), is something of an annual adventure. Last year, the Colts ranked 28th in the N.F.L. in rushing attempts and 29th in rushing yards. This year is even harder to forecast because their offensive line is in flux.

The Colts are so dependent on Manning and the points he produces — last year, the Colts ranked fourth in the league in points scored, averaging 27.2 — that they were able to win 10 games and their division last season even though they ranked 23rd in points allowed, with 24.2. That is especially bad news considering that after a Week 2 game against the Cleveland Browns, they play one of the N.F.L’s perennial superpowers, Pittsburgh Steelers.

The Colts have been fortunate for nearly 15 years to not have to think about a future without Manning. But if he is not back by the time the Colts face the Steelers, the glance ahead will be unavoidable. And probably painful.