Anthony, it's been an issue since you were drafted. 

As Jimmy D once said:

Still,
it was shocking to hear Detroit's Senior Vice President Jim Devellano
all but wipe his hands of Mantha when asked for an assessment of his
play. Very, very, very disappointing, Devellano told Keith Gave of Fox Sports.And I say that with a lot of sadness.Coming out of junior, we had such high hopes for him.May 5, 2015

If you were to ask Jimmy D to update his comment today, I think he would add another 'very' to his disappointment

produces like a 3rd liner at best, bad defensively. at 29, hes borderline replacement level. 

Supposed to be Franzen's replacement.

The Val d'Or games that I saw him play were quite revealing:  he barely churns his legs to get up to speed.  In contrast, Franzen's first three strides were unusually quick for a big man with a scorer's touch and he was a bit like Homer in that he was extremely hard to knock off the puck.

Not so with Mantha.  He was an opportunist and his release was too much for keepers at that level of play.  Nothing else stood out.