I only saw the highlights and round up on Match of the day2...but I kept an eye out for Mainoo.
When he had the ball, he was fine. When he didn't have the ball he was dire.
Casemiro was bad with the ball, hence getting pelters, but Mainoo was just trotting around whenever Liverpool attacked, when united lost the ball he didn't run back. He didn't seem to 'show' for the ball at all.
He is Rashford MkII, one or two 'wonder' appearances, lauded for the the England team, does bugger all for them, but he's the new poster English boy for the know-nowt's of the media, as Rashford's 'star' finally wanes and he is lambasted at increasing levels by his own fans for his one dimensional attack (run with ball, head down, into a blind alley/get lucky and score), and zero defence capability.