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, and his brilliance has covered a multitude of holes on their roster.

But when their 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 listed as doubtful for Sunday’s game against the Houston Texans.

More ominous is what else the team said: that Manning, who began having unexplained back soreness over the weekend, is being shut down in practice, and that the rate of improvement in his rehabilitation has slowed. Specialists are being consulted and tests are being completed, suggesting that Manning’s absence may extend further into the season.