Not an assumption, direct info from staff on the court last year. He was MIA for a bit, although if I remember it got downplayed somewhat, but my understanding is he had expectations coming in last year and didn't react well when things weren't going well early. Sounded like Forbes struggled for a bit understanding how to motivate him and that made it into some of his carrot/stick comments on coaching in today's era. To your own point, Marsh has gotten starting opportunities multiple times and lost them. It's not like the staff wanted to put Andrew Carr at the 5 but Forbes was forced to.
And nobody said he should be good because he's tall. To quote someone with practice access, if he cared half as much about the basics as he did dunking he'd have started all year. I appreciate the effort to assume the best in the situation, and he may be absolutely dialed in this year from an effort perspective. I just don't think fans criticizing him (without resorting to personal attacks) deserve to be rebuked.