Is it a mental problem? physical?
Both. No doubt he's had some injury/health issues, he's always banged up somehow and covered in muscle tape or what ever it's called. Mentally, ... immature I'll say. Not a bad thing because it's still very early in his development. It's very rare to have a 20 year old speaking, thinking, acting like a 30 year old and even when they do it doesn't necessarily translate to lasting on ice success (Gudbranson, Ekblad, look at a lot of the underage draft picks in the CHL).
Buddy's under the microscope now. There's no hiding away, there's no slipping past an opposing team's game plan. He signed his contract, he's in 'the show'.
Time to grow some hair on those Fleischklößchen