It could be, yeah. Though I'd also say Mainoo's been helped by the role he plays, and the lack of other options to cover for Casemiro's injury. Sometimes a gap opening up like that - similar to Rashford - is what a youngster needs to get that opportunity.
I'd also say, at this stage, he should be nearer the first team than Dan Gore (who I do rate as well). If Gore was to be loaned out now, I doubt he'd be linked to the likes of Sevilla, Lyon and Freiburg as Hannibal is. But I think that's the problem - Hannibal's at that stage, and level, where he's wanted by clubs like that so can hardly hang around United never playing and even dropping back into the U21's. Whereas Gore can for the second half of the season without anyone batting an eyelid, as he's a little way behind.
Personally, I'd have preferred to loan Gore out as this stage - probably to the championship - if one had to leave, and give Hannibal one genuine chance to prove himself either way. But I guess either is valid and, as you say, ETH sees them all in training (though, by that argument, you can defend all his selection choices on the basis that he sees more than we do).