Collie was slammed by a pair of Philadelphia defenders in the second quarter of Sunday's 26-24 loss to the Eagles at Lincoln Financial Field -- a controversial play that was ruled an incomplete pass on the field.
The receiver lay motionless for 10 minutes before being removed.
Caldwell said Monday Collie was alert and responsive after the hit, the Indianapolis Star reported.
"After he went in the locker room, he was able to sit up, alert, carry on a conversation," Caldwell said. "Obviously the doctors felt good enough about his situation to allow him to travel back with us on the airplane. I think that's a pretty good indication that things were trending in the right direction."
Collie told the Colts' medical staff his symptoms were improving, the coach said.
Some have called for Eagles safeties Quintin Mikell and Kurt Coleman to be fined for using an illegal helmet-to-helmet hit on the play, citing the NFL's recent crackdown on blows to the head.