I don't think United will go all in, it makes no sense, this is the first match and it is much more profitable to lose 1-0 now and fix everything at home than to rush into the attack now and lose due to counter attacks with a larger score.
Ajax is awesome. The top scorer in the Champions League this season has already scored two goals, but one in his own net

Winning in UCL is a pretty big deal especially in the 1st leg because it will makes the 2nd leg seems a little bit easier as the team wont be pressured that much to win but in this case, I dont see how United would win against Atletico so if we get to score a goal and get a draw on this game then its pretty much good enough for the 2nd leg.
Well what can you expect from Maguire, as usual this guy deserved to be benched forever