Barkov got hit hard and I’m sure it knocked the wind out of him. The hit didn’t result in a concussion which is supported by Hagel not hitting him in the head (although there could have been a whiplash effect but clearly not what would result in head injury since he didn’t miss any time other than what was mandated).
Hagel on the other hand did get hit in the head, twice, and is suffering from a concussion for him to already be ruled out, and if the Lightning do manage to get their act together and pull off a stunning comeback, it’s quite possible Hagel doesn’t play again this postseason or at least Tampa would have to go far. Those types of injuries linger usually.