Quality of the trifecta of Matic, Herrera and Pogba and when Ole took over, it was a relatively soft run of matches.
Cardiff (A), Huddersfield (H), Bournemouth (H), Newcastle (A), Spurs (A), Brighton (H), Burnley (H), Leicester (A), Fulham (A), Liverpool (H), Palace (A), Southampton (A)...then came Arsenal (A) - the first league loss under Ole for 3 months running.
Ole put square pegs in square places, etc. Then they lost their legs and lost matches when it was crunch time. Lost to Wolves away, beat West Ham, but then got embarrassed by Everton away, losing to City but then drew at Chelsea at home was devastating.
Matic becomes one year older, Hererra isn't replaced, Lukaku not replaced, Sanchez not replaced and a switch to two central mids leaves a big reliance on Martial, Rashford, James, Lingard, Pereria, Mata and Greenwood to generate 50+ goals between them at very minimum.
Since March 2, 2019 when United scored 3 goals against Southampton, United have only score more than 2 goals in a single PL match 1 time (4 v Chelsea) in the past 16 PL matches under Ole. 6 straight weeks this campaign with 1 goal or less (clean sheeted against West Ham).