Worry about nothing.
He is a rookie , learning , recovering from a very bad sprang .....which he may be trying to nurse and not mess up again .....which most likely caused the other stress on the Achilles in the same leg.
Plus nobody here knows what is exactly being asked of him every night......I suspect they are working on specific areas with each player ....according to HOW they see his development .....not what the Celtic blog thinks

They wanted Sully on the court shooting threes , so I imagine it's the same for Marcus , they want him to be a decent THREE point threat ...to do that he needs to take them in a REAL game.
Driving to the bucket may be easier for him to learn or ajust I think they want him to practice the three and get confidence ...like KO and Sully too.
It seems the powers at be want to develop the players first and win second .
I think in years ahead we'll get to see plenty of Marcus driving the lane.
I never seen such a panic as people have . He isn't Fab Melo .
Give the man a chance to learn and grow up ......of course if your only 14 teen yourself then you wouldn't understand