Slow starts definitely happen, but the most concerning thing to me is that Mantha doesn't create any offense by himself. Now that his lack of effort has moved him from Larkin's line, he's with Nielsen or Ehn, two guys who won't be able to create offense for him.
I think Mantha is more of a 45 point guy rather than a 60 point guy. Right now (and portions of last year) he couldn't be trusted in OT and didn't play on the 1st PP, this is very concerning for a 24 year old since his prime statistical years are basically over. Mantha's 48 points last year seems like it'll be the high end. In a league where speed is generally the deciding factor, Mantha doesn't skate enough to be able to be a big time contributor.