Louis van Gaal has spent close to £200m since taking charge at Old Trafford but still appears to have gaping holes in the squad.
The decision to play winger Ashley Young at left-back against Arsenal, and the continuing selection of Daley Blind at centre-half were both exposed by the Gunners' blistering start.
Blind played most of his first season at United in a deep midfield role and a lack of pace and strength were both in evidence against Theo Walcott and Alexis Sanchez.
Up until now, Blind has done well at centre-half but, as feared, did not have the attributes to play at centre-half against a devastatingly quick, high tempo counter-attacking outfit and got found out in our first true test this season.
Sanchez against Ashley Young was a complete mismatch as the Gunners ran United ragged in the opening exchanges.
Even Chris Smalling- rightly lauded for his early season form- was too slow to close down Sanchez as he bent in the third goal after easily outwitting Matteo Darmian- another previously impressive performer whom had a bad day on this occasion.
In front of our disorganised, bewildered back four, partnering Bastian Schweinsteiger and Michael Carrick together also proved to be the wrong call.
Carrick, 34, and Schweinsteiger, 31, are both masters at keeping possession and dictating play, but in an opening 15 minutes in which Arsenal completely dominated, their lack of pace against the counter-attack was brutally exposed and punished.
Morgan Schneiderlin- left on the bench-may have been a better option to break up play with pace to push forward as Arsenal's trio of Aaron Ramsey, Santi Cazorla and Francis Coquelin totally bossed midfield.
Antonio Valencia- introduced as a half-time substitute, could have been better at right-back (with Darmian on the left) to deal with the rampaging Sanchez.
The Ecuadorian is both quick and strong in the tackle and has at least played full-back on previous occasions, so tasking him with shackling the Chilean seemed to make more sense than an out of position left-footed attacking player.
The anonymous Memphis Depay- who surely must be playing for his place now- and Darmian, who was also disappointing, were hauled off at the break in place of Valencia and Marouane Fellaini, whom belatedly stiffened the centre and contributed to a better second half performance.
Anthony Martial was our only bright spot on a desperate day and came closest to scoring- and Schweinsteiger also forced a good save from Cech- but the damage was already done.