INDIANAPOLIS -- The unthinkable suddenly seems possible in Indianapolis -- opening the season without Peyton Manning.
The four-time league-MVP who has never missed a start in his NFL career is doubtful for Sunday's game at Houston because of lingering soreness in his back following offseason neck surgery. If Manning sits, it would be the first time the NFL's active iron man missed a meaningful game after 227 consecutive starts including the playoffs.
Manning's triceps strength has plateaued and he doesn't have the arm strength right now, sources told ESPN senior NFL analyst Chris Mortensen. Manning also had some back soreness that worsened during the weekend.
The Colts currently have no plans to attempt another surgery on Manning's neck, a source told ESPN NFL Insider Adam Schefter.
"I hate it for Peyton," said Pro Bowl center Jeff Saturday, one of Manning's closest friends. "You want him to be healthy, you want him to be back, but you want him to take care of it, too. It's sad when players face tough times, but I know he's strong enough to get through it."
The question is whether the Colts can survive without their franchise quarterback, whose streak stands second in NFL history to Brett Favre's among all QBs.
His 208 consecutive regular-season starts are the most among active players. If Manning can't start Sunday, Tampa Bay Buccaneers cornerback Ronde Barber, who has started 179 straight games, would take over the top spot if he starts Sunday against the Detroit Lions.
Manning has been the Colts' only starting quarterback for the last 13 years. Over that span, the Chicago Bears have started 17 different quarterbacks, the most of any NFL team. Every team other than the Colts has had at least three different starting quarterbacks during that span.
Team officials have not said how much longer it could take Manning to recover, leaving that decision to Manning and his doctors.
Over the years, life without Manning has been pretty dismal.
The only time he missed a regular-season snap because of injury, against Miami in 2001, backup Mark Rypien fumbled. The Dolphins recovered, then drove 59 yards for the winning score. Manning returned on the next series with a bloody mouth. He was later diagnosed with a hairline fracture in his jaw.
On the rare occasions when Manning has played sparingly, in the preseason and meaningless late-season games, the Colts have not fared much better.
The goal now is to change that trend.
"If anything, we want to go out there and step it up," Pro Bowl safety Antoine Bethea said. "Of course it will be different (without Manning), but it's one of those things that if it happens, everybody else has to step up. It's next man up."
That's been true at every position but one over the past 13 years in Indy. And now the Colts' longtime mantra will be put to its most challenging test.
Manning had surgery to repair a nerve May 23, but the recovery has taken longer than the expected six to eight weeks that would have put back on the field for the start of training camp. Instead, Manning started camp on the physically unable to perform list and wasn't activated till last Monday.
He did limited work at practice last week, which led to complaints about back pain. The team issued a statement Monday saying that team doctors re-evaluated Manning over the weekend and instructed him to stop practicing while he undergoes more tests.
No additional surgery has been scheduled.
"At the conclusion of the diagnostic process, if there are any new developments in the prognosis which we outlined for Peyton at the start of training camp we will report them," the statement said. "As of now, Peyton continues to deal with a complicated neurological recovery, the end date of which is unpredictable."
Losing Manning for any time, even one week, would be a shock to Indy fans. Not only has he never missed a start, he's rarely missed practice.
Manning sat out one week of training camp in 1998 before signing his rookie contract. It was a decade later when he missed all of training camp in 2008 because he underwent surgery twice to remove an infected bursa sac from his left knee. The only other time his playing status was in doubt was 2001 after he was injured at Minnesota in a preseason game.
Somehow, he's always made it back for game day.
"I'm sure it's killing him," receiver Anthony Gonzalez said. "I've not spoken to him, but knowing him, how hard he's working, how many hours he's put into it, I'm sure it's hurting."
The last quarterback other than Manning to start for Indy was Jim Harbaugh, now the San Francisco 49ers coach.
Many who know Manning well aren't counting him out yet.
On Monday, coach Jim Caldwell urged caution to those already writing the end of the streak.
Former Colts coach Tony Dungy, now an NBC analyst, told viewers recently he expected Manning to play unless he was "dead." Harbaugh had been thinking the same way before the latest twist in this saga.
"Watching Peyton Manning, the pro, the competitor, I'm not going to take your word for it that he's not going to be in there for the first ballgame," he told reporters Aug. 25. "So the streak may indeed just rage on."
Still, the Colts are preparing as if the second-longest starting streak by an NFL quarterback will be history after Sunday.
Sixteen-year veteran Kerry Collins came out of retirement, presumably to back up Manning in case this happened. Collins has spent the last 12 days learning the playbook and splitting work with Curtis Painter at practice.
Collins said that while he's comfortable running the Colts system and familiar with Houston's defense from his days in Tennessee, it would be unfair to expect anyone, including a former Super Bowl starter, to deliver a Manningesque performance.
"There's no way I can replace someone like Peyton and what he means for this team and this franchise," Collins said. "But I will bust my butt to get ready and I hope the guys see that."
Team owner Jim Irsay said he anticipated Manning returning for the start of training camp.
But after signing a five-year, $90 million contract in late July, Manning was placed on PUP and didn't return to practice until last week.
Manning has repeatedly said the 4½-month lockout delayed his rehabilitation because he couldn't work out with team doctors.
When he returned in 2008 after missing all of camp, Manning struggled during the first half of the season. He eventually led Indy to nine straight wins and back to the playoffs, picking up the third of his record four MVP awards.
Could he take a snap to keep the streak alive? Sure.
But Manning ruled out that possibility last week.
"I have to be able to competitively play," he said then. "I have too much respect for football. I've got to be able to compete and to help my team win, and that's what's fair to the team, fair to myself."
And right now, it appears, that means playing with someone other than Manning behind center for the first time since December 1997.
"He loves to play, loves to compete, loves to practice, and any time he can't, it's tough," Caldwell said. "That's who he is. He's always been a hardworking guy. He doesn't miss days -- offseason, in-season, you name it -- he's always there. I think that's the commitment to the sport he's made since he was a little guy."
Information from ESPN senior NFL analyst Chris Mortensen, ESPN NFL Insider Adam Schefter, ESPN Stats & Information and The Associated Press was used in this report.