I've stated before that I'd be inclined to give McCarthy the opportunity to rekindle some of the form shown in the first season.
For as you rightly mention, he's not the only player to have dwindled under Martinez and his shoe-horning approaching within the last two years.
Yet, if Celtic's interest is genuine and they're willing to offer a fair amount for a permanent deal then Koeman may see it easier to offload him.
Talk of a loan deal however means he'll most probably be staying at least in the short to medium term.