"NFL Season opens 2nite!We had a good practice yesterday and r guys r fired up 4 the season.#18's out for awhile,but compete,we will/BELIEVE," Irsay wrote on his Twitter page.
Coach Jim Caldwell said Thursday that the league's four-time MVP has completed the doctors' tests and team officials now are trying to determine the best course of action to get Manning back on the field.
Caldwell spoke to reporters several hours after Irsay tweeted.
"It's obviously accurate otherwise he wouldn't have tweeted it," Caldwell said. "In terms of the timeframe we're talking about, I think he's also stating we don't know. None of us know. It is a little bit in flux at this point, but I think we will have some more clarity here shortly."
An announcement, Caldwell said, is likely to come soon.
"I guess everyone's definition of 'awhile' is different," defensive captain Gary Brackett said. "I know things happen in the NFL that can be very strange. You can see guys get better instantly or you can see it go the other way instantly as well. So in this league, I think everyone is day to day and that's how I'm viewing it."
Manning will be in street clothes, still recovering from neck surgery, when the team opens the season at Houston on Sunday. Kerry Collins will start in his place.
"To say I am disappointed in not being able to play is an understatement," Manning said in a release from the team. "The best part about football is being out there on the field playing with my teammates. It will be tough not to be out there playing for the organization and our fans. I simply am not healthy enough to play, and I am doing everything I can to get my health back. The team will do fine without me, and I know for sure that I will miss them much more than they miss me."
Manning had neck surgery to repair a nerve May 23, but the recovery has taken much longer than the expected six to eight weeks that would have put him back on the field for the start of training camp.
He signed a five-year, $90 million contract in late July, but started camp on the physically unable to perform list and wasn't activated until last Monday.
He did limited work at practice last week, which led to complaints about back pain. The team issued a statement saying that team doctors had re-evaluated Manning and instructed him to stop practicing while he undergoes more tests. No additional surgery has been scheduled.
On Wednesday night, Irsay debunked rumors that Manning had recently undergone another surgery on his neck.
"Peyton didn't have a medical procedure last sunday,we'll have more info 2 add clarity 2 situation soon," he wrote on Twitter.
Manning's 208 consecutive regular-season starts are the most among active players. When he officially misses Sunday's game, 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.
The last quarterback other than Manning to start for Indy was Jim Harbaugh, now the San Francisco 49ers coach.
The player who can empathize most with Manning is running back Joseph Addai, who injured a nerve in his left shoulder Oct. 17 against Washington and then missed the next eight games.
There were times, Addai recalled, that he would wake up during the night with sudden pain. There were other times that he couldn't hold up a microphone or the ball would drop out of his hands with a slight bump.
Addai figured the ensuing bye week would give him enough time to heal, but it took him more than two months to get back into a game and he still didn't feel 100 percent until this season.
"After awhile it came back, but you don't really know when it's going to come back," Addai said. "It's frustrating."
Addai said Manning has asked about the experience, something the two have discussed at length since players reported to camp July 31. Addai said he has not yet caught passes from Manning at full speed and he can't say where Manning's recovery is right now.
Instead, he's worried about playing the Texans without Manning.
"You know how important Peyton is," Addai said. "I think everybody has to step it up."
Information from The Associated Press was used in this report.