Ballard said Luck first suffered a calf injury and has developed an ankle issue while attempting to recover from that initial ailment, according to The Athletic's Stephen Holder.
However, the team isn't ready to make a decision about the quarterback's Week 1 status, ESPN's Mike Wells reports.
Ballard did confirm, though, that the quarterback is unlikely to suit up during the preseason, according to Wells. He'll need to get in a certain amount of practice time in order to be cleared for the season opener.
Colts owner Jim Irsay said earlier Tuesday that Luck's absence from training camp has been due to a bone injury in his lower left leg.
Luck went through a similar ordeal two years ago with a shoulder injury that was initially believed to be minor but wound up sidelining him for the entire campaign. Ballard, however, insisted this ankle issue is a different scenario.
"This is not 2017," he said, according to Holder. "I've thought about this a lot ... We've talked a lot about building a team."
Indianapolis battles the Los Angeles Chargers in Week 1, which is less than one month away.