The fact that they announced it early tells me it's somewhat serious for Mitchell. Meaning he's probably done for this series regardless, though it may not matter anyways since they are down 3-1. It's felt the same way with Jarrett Allen, no concrete updates and always "questionable".
But still, I would not fool around. Last thing you need is to have this go back to Cleveland on Friday night and then it turns out Mitchell and/or Allen return. Got to take care of business tonight