Mantis feels his jaw go slack, but he gets his composure back by the time Jason starts to stand again. Definitely dead. Definitely animated. Definitely a problem. He needs to get away, but there's no telling how well Jason can follow him, and he can't use proper violence to try and maim the strange zombie.

He focuses on immobility, trying to pin Jason to the ground from a distance so he can't even lift a foot off the ground. Can't throw giant rocks if you can't reach them. Right?