I just watched it several times, and it was definitely flag-worthy. We’ve seen penalties assessed for plays that were far less clear than this one.
When the Colts sent out news that Gonzalez landed on injured-reserve, they indicated they were upset with the play by saying he suffered the injury “when being tackled out of bounds.”
It’s a freak injury, certainly involving bad luck. Perhaps Gonzalez didn’t brace for a tackle in that spot the way he would have on the middle of the field, because he assumed the play was over. It’s difficult from the TV footage to determine just how it happened but it’s clear as he gets up and crossed the field to Indy’s bench that something is wrong.