#1 Poor conversion
In a game of high stakes, it goes without saying that teams need to take full advantage of their openings. Chelsea made the most of their minimal chances but despite having the majority possession and almost 3 times as many shots, United failed to find the back of the net.
They were in complete control of the game in the second half as Chelsea were more than happy to sit back and hit United on the counter. Courtois had to make a couple of brilliant saves but the United players simply had to be more clinical.
All their domination failed to bear fruit in the end with Chelsea running out as winners and the story could've been very different if United had that extra bit of composure of ruthlessness in front of goal.