It's nothing to do with luck. If you're so tactically inept as a manager as to try and hold on to an aggregate goals win at home against relegation fodder in a season defining game, then you make you're own luck by being useless at your job.
He's just isn't a good manager. It's been glaringly obvious for a decade - his nous in the transfer market as a glorified director of football propelled us up the league as the standard of player just dictates that you can't fall so far, but at the level he's at now you've got to know how to use the players effectively to win.
World class director of football, an abysmal failure of a manager.
That's nonsense.
As a manager he got us from bottom half of the table to consistent top 6 challengers on a shoestring budget with a few European trips along the way. Regardless of whether he's tactically inferior to other managers at the highest level, he's never a failure as a manager.
He may fail at United, but very few coaches are cut out to manage one of the biggest teams in the world.