In a sense, yes - but in another sense, he's better suited than a lot of managers to take over a big side having achieved success with Ajax. They're not as big as United, but they're the biggest club in Holland and managing big clubs comes with some additional requirements for managers: expectation is much higher and the media scrutiny is more intense.
Aside from looking like he's got the goods tactically, I think his time at Ajax is fairly good preparation for his future at United.
I just think United fans and the board will need to show some patience with him. It looks like it took him a few months to really get going at Ajax - and out of this current United squad, I think he'll want to be replacing a lot of players to get them playing in his style. United can probably afford to sell a shitload of players and buy a whole new team - but there's also issues in doing that as well with team chemistry and whatnot.
I think he might get a season (or two) to really get going - and I dunno if United really have that kind of patience with a manager that's not one of their own like Solkjaer was. But they've needed some sort of long term vision for a while now and I think if they stick with him they'll end up a very good side in a few seasons.