The NFL has spotters at each game as part of the league's heightened awareness about concussions, but Henderson's condition went unnoticed.
"There were no indications after the game, Monday, Tuesday and then Wednesday came after practice and (he) just didn't feel right," Vikings coach Leslie Frazier said. "That's when they decided to take him through protocols for concussions."
Henderson has 26 tackles, two sacks and four tackles for loss – leading the Vikings in all three of those categories.
He went through more of the NFL-mandated post-concussion tests on Thursday, but the Vikings don't know if he will be available for Sunday's game.
"We're trying to determine if he's going to be ready to go by Sunday. We'll know a little bit more tomorrow," Frazier said.
"There was nothing to indicate there was something going on until he came out (for practice) on Wednesday."
Henderson showed up at Wednesday's practice with a helmet, but he didn't participate in any of the team's 11-on-11 drills, according to the official injury report.
If Henderson is unable to play, free-agent acquisition Marvin Mitchell would take his place. A seventh-round pick out of Tennessee in 2007, Mitchell played four seasons with the New Orleans Saints, making two regular-season starts and two starts in the playoffs, before joining the Miami Dolphins last year.
"He's been in ballgames as a starter. He's been in ballgames as a backup. He's played a lot of football so everything that happens out there is not new for him, so that gives me comfort," defensive coordinator Alan Williams said. "And when he's been in ballgames, he's performed well."
While Henderson had another solid outing on Sunday, his mistake in coverage left an enduring blemish on that performance. With the Vikings trailing 10-6 late in the second quarter, the Colts drove into field goal position and likely would have settled for three points if Henderson hadn't allowed receiver Reggie Wayne to get behind him in a zone that was Henderson's responsibility to cover deep.
"I misplayed the ball, I misjudged it a little bit, so hopefully there are quarterbacks out there, watching it, and they think that they can come in and try to get some on me, too, and I got a little surprise for them if they want to present that to me," Henderson said Monday. "So, like I said, I watched the tape and I learned from it, and I get better and I move on from there."
Williams said Henderson was playing well besides that glaring mistake.
"It's a shame because he played an outstanding ballgame. He really played well," Williams said. "That doesn't take away the touchdown that happened, but he had a few tackles for a loss, he was in there on the sack. He was playing an outstanding football game, doing well. Doing extremely well."
Williams said he wasn't sure if Henderson would be available Sunday. If not, Jasper Brinkley would join Chad Greenway in the nickel defense, when Mitchell would come off the field.
Tim Yotter is the publisher of Viking Update. Follow Viking Update on Twitter and discuss this topic on our message boards. To become a subscriber to the Viking Update web site or magazine, click here.