Its also blatantly true that the squad isn't good enough -I'm all for criticizing him for certain things (like negative tactics and picking fellaini on Tuesday night for a must win game) but DDG is the only player in the squad that JM didn't bring to the squad that's proven quality (arguably Martial and Rashford, while both are having very good seasons their form is very up and down, with their ages this is to be expected).
I'm not expecting (or wanting) every squad player to be a super star - this rarely ends well when it does happen - but even the squad players are far too below the quality required for Utd, even for the occasional "fill in" game - but its a direct result of the minimal investment SAF made at the end of his time at the club followed by the two disasters of managers in Moyes and van Gaal.
Valencia and Young aren't really good enough to be anything other than back fill, the fact they are 1st choice and have been accepted as such is just the start of where things go wrong. When the likes of Pogba and Sanchez drastically misfire on any given day in their performances in general even though they are probably paid between them more than 1/2 of the squad put together, its not surprising when others under-perform.
JM made a very good point about the core of Pep's side predating him