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INDIANAPOLIS – There’s no reason for concern. Yet.

But there’s always anxiety when a front-line player misses practice on a Thursday prior to a game with an injury issue that apparently cropped up the previous day.

And there’s always anxiety when that player in the team’s two-time, first-team All Pro guard.

Quenton Nelson was the only player who missed Thursday’s practice as the Indianapolis Colts ramped up work for Sunday’s opener at Jacksonville. The culprit: a back issue.

Nelson was a full participant in Wednesday’s practice.

The red flag regarding his absence Thursday is because the 2018 first-round pick almost never misses time in practice or a game.

Nelson has started all 34 games, including the playoffs, and has been on the field for 1,181-of-1,213 offensive snaps during the regular season. He missed 31 snaps in the Colts’ week 16 win over Carolina last season after sustaining a concussion.

The concussion kept him out of one practice heading into the season wrap-up at Jacksonville as he went through the NFL’s concussion protocol, but he was cleared and played against the Jaguars.

Increasing the anxiety over Nelson’s status is the lack of proven depth behind him.

Fifth-round pick Danny Pinter is the backup at center and both guard spots. The only other guard prospect is Jake Eldrenkamp, who’s on the practice squad and has yet to appear in a regular-season game.

Nelson isn’t the only member of the Colts’ offensive line dealing with an injury.

Left tackle Anthony Castonzo (oblique) and center Ryan Kelly (knee) have been limited the past two days.

This is not the way the team hoped to enter the season. The offensive line is considered one of the best in the NFL and the Colts’ five starters were able to start all 16 games for the first time since 2000.

Also, defensive end Justin Houston was limited Thursday with a calf injury.

Rookie wideout Dezmon Patmon, who did not practice Wednesday with a knee issue, was back on the field Thursday.

You can follow Mike Chappell on Twitter at @mchappell51.

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