Chris Mortensen of ESPN reported Luck will begin a throwing regimen upon his return from Europe, which could indicate surgery is necessary.
Luck, 28, missed the entire 2017 season because of a slow recovery from January shoulder surgery. He was initially expected to return for the beginning of the regular season, but persistent pain in his shoulder has kept him from even throwing a ball for months.
The Pro Bowler went to Europe last month for treatment that is not available in the United States. Colts owner Jim Irsay said earlier this week no additional surgeries are planned at this time.
"He's doing great," Irsay told NFL Network's Ian Rapoport. "He's doing well. [We're] disappointed obviously it's taken as long as it's taken. You know, medicine and the way it goes. But there hasn't been any unusual setback. We didn't find out anything ominous, something we didn't know about or anything like that. It's just taken time for him to go through his whole aspect of rehabbing and progressing and working through the soreness.
"He's still in Europe but he's coming home very shortly. No additional surgery at this point planned."
Luck made three Pro Bowls from 2012-14 and led the NFL in passing touchdowns in 2014.