Not that I disagree with either of you, appointing Mckenna would be a massive gamble, but I think it's quite harsh to judge him as having failed at United when he was just the assistant coach. A few weeks ago, on The Athletic's Totally Football Show, they were praising Mckenna for his work at Ipswich and Carl Anka was one of the panelists and he was talking about how Mourinho and Ole both have an incredibly high opinion of Mckenna. He also said that Mckenna's main mandate/role under Ole was working on United's defensive organization and that both Ole and the players found him to have very clear ideas which he was able to instill in training.
I also do remember there were a lot of reports in the summer leading up to Ole's sacking that many of the players thought that the training at United was "too British" and too unorganized on the ball, though I can't be bothered to find the reports right now. But it was also reported that, as @VP89 said, the players thought well of Mckenna but found him too school masterly.
Based on all that, is it not reasonable to conclude that -
1. The report that the players complained that the team was too unorganized on the ball was probably true. I distinctly remember, after a number of embarrassing losses, De Gea saying in the post match interview something along the lines of 'we don't know what we're supposed to be doing when we have the ball.'
2. The players generally thought highly of Mckenna. I mean both Ole and Mourinho's praise seems to corroborate that. He was highly rated at Spurs when he joined United's youth coaching setup and generally did well with our youth teams, didn't he? He's also done brilliantly at Ipswich, so there's obviously real coaching ability there.
3. He may not be to blame for the players looking disorganized on the ball if his job was to organize the team defensively. Like, I don't remember United being notably brilliant or bad defensively under Ole but the xGA numbers seem mostly fine, if not particularly impressive. But, by all accounts, Ipswich aren't too great defensively (I haven't seen tham at all so can't really comment) either so his defensive ideas are maybe just not that great? I do remember United being dreadful defending set pieces under Ole.
4. At Ipswich, he's had time to implement his own vision on the ball and Ipswich have scored bucketloads. So maybe at a side with better players, he can scale those attacking ideas further up?
