Caldwell told reporters Decker suffered a shoulder injury in practice last week.
Caldwell said Decker will be out until at least training camp and did not rule out the chance that he might miss the start of the regular season.
After selecting Decker with the No. 16 overall pick in the 2016 NFL draft out of Ohio State, the Lions started him at left tackle in all 16 games last season, as well as their playoff contest, and he played every snap.
With the 23-year-old on the shelf, Detroit's offensive line depth will be tested.
The Lions signed right tackle Ricky Wagner and guard T.J. Lang in free agency, but they allowed tackle Riley Reiff to depart for the Minnesota Vikings.
While Decker is out, Cornelius Lucas and Joe Dahl will vie for playing time at left tackle.