He got it wrong. It happens. We've come a long way from where we were, to the point where many were optimistic about a win yesterday, which hasn't happened in a long time.
What is most important is that we improve from this going forward. You could argue that this has been coming, though not in the statistical 'we were due a loss sometime' way. In a lot of matches recently (With the exception of Barcelona who were inexplicably unimpressive over the two legs) we have been coasting through at least half of a lot of these matches, usually the first half, and then doing just enough to get the win in the second half. Which is great in that we've been able to get the results and we've looked good while doing so, but there was always the chance that a match would come along where we weren't able to just switch it on in the second half and get the win. Unfortunately that ended up being yesterday and that was the result.
All we can do now is pick up and move on, and hopefully this doesn't derail the season. If this was the squad from two years ago I would have guessed that that was the more likely outcome, but we have some real winners in the team now who, along with ETH will hopefully get things going again before long.
As for the team itself, to my non football mind, I think there are a couple changes we can potentially make. Firstly, we should switch to a midfield 3. As good as Cas and Fred have been, we rarely have prolonged periods where our midfield is in control of the game, so there's always a chance we could be gotten at. And Bruno isn't a CM either. I would play Cas, Fred and Sabitzer, Rashford on the left, WW up top (Due to lack of options, not on merit) and Bruno wide right. Get some stability in the team and secure a few wins in a row to rebuild the momentum.