I understand your thinking. If he did have one, would the Leafs put him in red? They don't want anyone on the outside knowing what his injury actually is.
Some are saying he's fine because he looks fine skating. He can shoot it. What does that leave? His head is something he could do everything he can normally do. Not saying it is his head, but he wouldn't look any different if it was. Head injuries are really mysterious. You can hit your head, in a minor way, and suffer severe issues from such a minor bump. You can smash the heck out of your noodle and be just fine a few days later.
Maybe, he got hurt a few games ago? Maybe, in practice? On his off-time? We know very little. Keefe has poo-pooed this injury leading most everyone to believe he'd miss just one game. Now, he's missed two, and stated he's not sure what lineup will look like on Thursday. He's not saying Auston will play Thursday, as of last night.
Some will believe he's fine, regardless. Some will believe he's really hurt badly, regardless. I simply don't know either way, but missing two games and potentially a 3rd, per Keefe's uncertainty about Thursday, starts putting more weight on it's more than minor.