Collins did not play in the fourth quarter of Sunday night's game against the Pittsburgh Steelers because of concussion-like symptoms.
"He's still being evaluated, but he had those kinds of symptoms," Colts head coach Jim Caldwell said on Monday. "The doctor's did what they're supposed to do in that regard, which is to remove him from the ball game and continue to evaluate him. He's still in that process right now."
Collins, signed into a starting role in late August when it was revealed Peyton Manning had not been fully recovered from offseason neck surgery, took several big hits and was clearly dazed on the sideline.
Curtis Painter, who had attempted just 28 career passes coming in, relieved Collins on the Colts' first possession of the fourth quarter.
Collins completed just 13-of-29 passes for 93 yards, but exited when the game was tied, 13-13. The Colts eventually lost, 23-20, to fall to 0-3.
Manning, whose status for the rest of the season is still in jeopardy, was in the offensive coordinator's booth during the game.