Here is a quick overview of his projected play style courtesy of Future Considerations:
Mantha has good bloodlines and impressive size. Handles the puck well and makes a strong breakout pass. Uses his reach and strength to defend. He needs to work on his transition and overall mobility as well as improve his physical play as he comes off as somewhat soft.
Mantha was one of the first players that I was able to see this season when the Musketeers traveled to Hackensack, NJ to play in the first annual USHL Atlantic Challenge. In that viewing, I was excited about what he had shown offensively and somewhat impressed by his defensive efficiency, but I clearly noted his lack of physicality which burned him a couple of times in the game. And as the season progressed, physicality still escaped him
As usual, if you interested in reading up on some background information on Mantha, Yahoo Sports has you covered.