That doesn't really explain it. The top level of the CL isn't worse than the mid or bottom level in the EPL, and he managed that quite well. You could say it's a different kind of football, but he's also not reproducing results against European opposition or the top EPL teams, which don't play that differently from top CL teams.
Also, even if the issue is that his approach doesn't work in the EPL, you'd at least expect to see a consistent system that then fails to consistently win games in the EPL. But United's football is often messy and disjointed. Like, I don't think 'he's not good enough for the EPL' explains why he sometimes can't even seem to get the basics of coaching right.