I see it rather the opposite way - this was the most deflating defeat of the season. Because there is no longer an upwards trend - that came as the third straight game of declining performances, which again comes on the heels of a fairly protracted period where it looked like the revival was on, with a lot of fundamental things working very well.
West Ham was the first game since City where we were looking less good than previously, across the board really. Against Real Sociedad, our attacking game reverted back to the abysmal level we had during our first recovery (Southampton, Leicester etc) and our midfield control was gone. Yesterday, defensive solidity collapsed too.
In short - we're pretty much back where we were after the City game. Progress has not proven sustainable, so for now the revival is not on after all. Maybe we can get back on track, only that's not going to be until after the World Cup. Hopefully we can make a start at least against Fulham. With our current squad depth, an extra round in the EL and two deferred PL games in an already gruelling schedule I would not shed a tear if we take the opportunity to crash out of the Carabao in midweek.
What happened? Injuries and suspensions played a significant part, obviously. And it's hard not to note the fact that Ronaldo started all three of those games and performed feebly in all of them, although it is also hard to imagine that this could have such a huge effect in itself. And none worse than the game for which he was named captain. Maybe that says something.