I don't think there was anything wrong with Manchester United's defense today, but they just failed to properly anticipate Caicedo's kick which came from a loose ball. Goals with this scheme are very difficult for any team to anticipate, even if they have good defenders. Manchester United just needs to increase their goal productivity and not waste many opportunities, that is the problem in this match in my opinion.
Actually this is still quite good, at least, under Ruud, Man United maintains its performance and only shares points, not defeating I know this is certainly not that satisfying, but this is much better than some of the series of defeats experienced by Man United. especially this time the opponent is Chelsea who also often troubles big tips.
Source: Manchester Utd vs Chelsea
Their defense has also started to improve, although it is not yet optimal, at least there are some saves made by their defense.
Here are the players rating of this match.