They should have called someone else up to be a long man.
I feel like they weren't planning on him being the long man (well, in an emergency he would've pitched). I think they intended for him to get the start tonight when they called him up Monday, but listed him in the pen just in case something unfortunate happened, since McFarland wasn't available for at least 2-3 days. Then over the course of the week, they realized just how much trouble the Yankees have had with lefties recently, combined with not needing to use too much of the bullpen due to the good starts, and decided that McFarland is the better option for tonight.
It is a lack of foresight, I'd agree, if that is indeed how it worked out. Should have known the Yanks troubles against lefties, planned for McFarland to get the nod, and then called someone else up for the long man role. Definitely seems like a waste to have Gausman up here for 5-6 days and not pitch. I don't think they would have called Gausman up to solely be the emergency long man had the plan all along been for McFarland to get the start tonight.