Taking calls on any player is such an ambiguous statement, it could be nothing more than due diligence on the Knicks part. Mitch is a weirdo, but he's also on one of the most cost effective contracts in the league, it'll be hard getting back equal value for him. I wouldn't consider Nick Richards equal value for Mitch either, Mitch is a far more impactful player.
I see Mitch sticking around, at least for next year before he's actively shopped. As injured as Mitch usually is, his skill set is probably the second hardest to replace on the team, and a big chunk of our offensive identity is centered around offensive rebounds, which he's literally the best at doing in the league. He's still one of the best defensive bigs in the league too. With Mitch I think we're probably the second or third favorite to win it all, without him we probably take a significant step back (especially when you consider that we lost our second best rim protector already, and we lost our second and third best rebounders this off-season too) .