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INDIANAPOLIS, Ind. – Imagine, the Colts’ most indispensable player being on the ominous-sounding physically unable to perform list as training camp opens.

Imagine, one of the NFL’s upper-level quarterbacks being away from his teammates, working with trainers and the rehabilitation staff, trying to get himself to the point he can actually practice.

Imagine the angst permeating the team’s fan base – and perhaps some of the players – as the backup quarterback runs things until the guy is back under center.

Imagine not knowing exactly when that might be.

That’s the overriding storyline as the Colts open camp Saturday at their Indiana Farm Bureau Football Center complex.

Andrew Luck will be on hand, but on PUP and still in rehab mode as he continues his comeback from January surgery to repair a torn labrum in his right shoulder. He began throwing last week and the expectation is for Luck to gain medical clearance for practice sometime in August.

As unsettling as the scenario is for a team hoping to return to the playoffs for the first time since 2014, it’s not exactly uncharted territory.

Remember 2008?

Remember the most indispensible Colt – Peyton Manning – being placed on PUP at the start of camp? He had an infected bursa sac removed from his left knee in mid-July and the subsequent rehab forced him to miss the bulk of preseason practice and all four preseason games.

“I’m disappointed I can’t be there today,’’ Manning said as the ’08 camp opened. “I’ve never missed a practice or a meeting or an organized activity in my whole career, and I’m disappointed I can’t be there with all the guys on the first day of training camp.

“But as the cliché goes, ‘Following doctors orders.’ My goal is to get up there as soon as I possibly can.’’

Manning also spent the entirety of the 2011 training camp on PUP while dealing with his multiple neck procedures that would eventually force him to miss the entire season.

But ’08 seems to offer a reasonable road map for what to expect with his successor. It ended with a 12-win season, seventh straight playoff berth and third MVP award for Manning, but only after a rocky start by the somewhat rusty QB.

We’ll rehash Manning’s ’08 PUP experience in a minute, but first a quick refresher on Luck.

General manager Chris Ballard insisted the team’s $140 million QB has suffered no setbacks and anticipates Luck will “come off of PUP before the start of the season.’’ He declined to address whether Luck might see action in any of the four preseason games – we’re betting against that – or whether he’ll be ready for the Sept. 10 regular-season opener in Los Angeles against the Rams – we’re betting he’ll be under center.

Under even the most positive scenarios, Luck will enter the Rams game having been throwing for two months and with perhaps two weeks of serious practice with the No. 1 unit. The coaching staff undoubtedly will be closely monitoring his workload.

“When we do move forward with Andrew and decide it’s time to get ready to go, he will have gone through the process completely where he’ll be ready to go and perform at a good level,’’ Ballard said. “Whether he plays in the preseason or doesn’t play in the preseason, that’s up to our doctors and trainers to tell us where he’s at and for Andrew to tell us where he’s at.’’

Luck is coming off one of his most efficient seasons. He passed for 4,240 yards and 31 touchdowns, and set career bests by completing 63.5 percent of his passes and averaging 7.8 yards per attempt.

But it’s beyond risky to assume Luck will simply pick up where he left off. He delivered his final pass Jan. 1 against Jacksonville, a game-winning 1-yard touchdown to tight end Jack Doyle.

We can debate how much practice time a veteran needs to prepare himself for when games matter – Luck is entering year 6 and has started 76 games, including six in the postseason – but extended downtime never is a good thing. That’s especially true when that downtime involves a quarterback going six months without throwing a football.

That’s where Manning’s ’08 experience serves as cautionary tale. And let’s keep in mind Manning enjoyed his normal offseason regimen before the knee issue interrupted his preseason. Luck’s entire offseason has been anything but normal.

The Colts had opened the three previous seasons with 9-, 9- and 7-game winning streaks. They opened ’08 and christened Lucas Oil Stadium with a 29-13 loss to the Chicago Bears. They rallied for an 18-15 win at Minnesota on the strength of an 18-point second half and Adam Vinatieri’s 47-yard field goal with 3 seconds remaining, but fell to 1-2 with a 23-21 home loss to Jacksonville.

The poor start couldn’t be blamed solely on Manning and the offense still searching for their timing and rhythm, but there’s no question he wasn’t himself. Consider the marked difference in Manning’s faulty start and eventual turnaround:

In the 1-2 start, Manning completed 59.1 percent of his passes and averaged 6.5 yards per attempt with four touchdowns and three interceptions. His passer rating: 73.1.

Over the remainder of the season, Manning led the Colts to an 11-2 record. His completion percentage bounced to 69.96 and his yards per attempt to 7.4 He delivered 24 touchdowns, suffered eight interceptions. His passer rating: 101.1.

Manning, like Luck, was a workaholic. Whatever happened on game day was a byproduct of practice, of adhering to a routine.

After missing the ’08 preseason, Manning realized he was heading into “uncharted territory.’’

“That’s kind of what it’s been all preseason for me,’’ he said. “I just hope to have a good week of practice this week (for the Bears) and get back into the flow of things and hope I can go out there Sunday and play at a high level.

“You’re somewhat in the unknown a little bit. To think you can just parachute in there on Sunday into the new stadium on the day of the Bears game and think you’re ready to go is ridiculous.

“Obviously I’m going to need some time.’’

And so will Andrew Luck.