Look at the squads they inherited/left behind:
SAF/Moyes 2013
----------DDG----------
Rafael-Rio-Vidic-Evra
Val--Carrick-Cleverley-Kagawa
-----Rooney--RvP--------
What this squad needs:
An all-round CM to complement Carrick, and a top winger.
Long-term a passer to replace Carrick, another CB once Rio/Vidic go, and an Evra replacement.
In terms of form:
Defence: Rio-Vidic had a good season, with Evans showing great potential, Jones and Smalling raw and in reserve. Rafael had a great season but there was no real RB backup and Buttner is just terrible.
Midfield: Carrick was magnificent all season, Cleverley was below-par. The only backup was Giggs and Anderson.
Forwards: RvP was god, Rooney was struggling but trying to adjust to playing 2nd fiddle, Welbeck showed some promise, Kagawa had a slow start with some promise. Valencia's #7 season was an eyesore, and Nani had a sporadic season with some big highlights.
Moyes/LvG 2014
---------DDG--------
Rafael-Jones-Evans-Buttner
Mata--Carrick-Fellaini--Welbeck
---------Rooney--RvP-------
What this squad needs:
2 top-class new CMs, a new winger, a new LB, and 2 new CBs (one first choice, one backup) and maybe a new RB to replace Rafael. Maybe Roo or RvP to be shipped out to make room for Mata.
Long-term the frontline was ageing fast and one or both needed replacing.
In terms of form:
Defence: Rafael had a stinker and Valencia was a terrible replacement RB, Evans had regressed and Smalling and Jones were sill backup, Buttner is and will always be terrible.
Midfield: Carrick, Cleverley and Fellaini had just had probably the worst seasons of their career.
Forwards: Rooney-RvP looked completely dysfunctional as they occupied the same space. Januzaj could occasionally provide a touch of class. Welbeck as a bit stagnant but the potential was still there. Kagawa had regressed completely, Nani hadn't played at all. Mata was efficient but not especially creative.
LvG/Mourinho 2016
---------DDG--------
Val--Smalling-Blind--Shaw
------Carrick-Fellaini------
Mata-----Rooney-----Martial
----------Rashford---------
What this squad needs:
2 top attacking players, a top-class CM, a CB backup/replacement, and a solid RB.
Long term: if the short term issues (Carrick, Fellaini, Rooney) are fixed, there's no ageing problem.
In terms of form:
Defence: Smalling had matured into a top PL CB, Blind had somehow formed an effective partnership with him, Shaw had a promising start cut short by injury. Val is not a CL-standard RB but ok as a stopgap. For backups, Rojo and Jones are awful, Darmian is good tactically but poor 1v1, and CBJ showed some promise.
Midfield: Carrick and Fellaini were okish, Schneiderlin and Herrera are good options but in bad form.
Forwards: Martial and Rashford are awesome prospects who had great seasons. Mata and Rooney were below standard but generating numbers. I forgot Depay and Young exist.