The modern game emphasises full-backs as never before. They are chosen as much for an ability to initiate attacks as their quality in halting them, yet they have become the primary target for tactics to undo nine-man defensive blocks.
A match that eventually met Sir Alex Ferguson's demand that "we keep on the coat tails" of Manchester City, while leaving Arsenal an unheard of midseason 18 points astern of a club they used to fight for the Premier League title, stressed that tactical point.
This was won and lost on the flanks.
When Arsenal were obliterated at Old Trafford at the beginning of this season Armand Traore and Carl Jenkinson lined up at left and right-back.
The former has since been sold to Queens Park Rangers; the latter has been relieved of his vertiginous promotion from League One football by a leg fracture.
Bacary Sagna, the club's only proven full-back remains sidelined with Kieran Gibbs, an individual few Champions League managers would trust as their sole specialist left-back, so two fine central defenders were asked to compensate here. Thomas Vermaelen and Johan Djourou were found wanting.
Manchester United's opener began with a wild clearance from Vermaelen, the stand-in on the left side.
Still the Premier League's most accomplished exponent of that position, Patrice Evra sprinted in to claim the 60-yard punt ahead of his opponent and shuttled it on to Nani.
A second quick pass put Ryan Giggs in behind Djourou - who had spent most of the half chasing air - and the midfielder's cross found Antonio Valencia at the back post.
A malpositioned Vermaelen watched that centre fly over him; the header past Wojciech Szczesny.
Multiple chances spurned ahead of it, United's decider came from the same wing.
Paul Scholes allowed Valencia, by then a makeshift right-back for the visitors, to round Andrey Arshavin, exchange passes with Park Ji-sung, and tee up Danny Welbeck.
Throughout the match, the understanding and effectiveness of United's wide players underlined Arsenal's shortcomings.
Djourou was replaced at half-time with Nico Yennaris, an 18 year old making his Premier League debut.
"It's been their problem for a few weeks," said Ferguson. "Without Sagna and Gibbs, who are natural full-backs, it's difficult for them."
United's big issue is a midfield almost incapable of 90 minutes sustained control.
On the back foot for most of the first 45 minutes, Arsenal began creating in the second half until a Laurent Koscielny-initiated counter allowed Robin van Persie to bring the fixture back to 1-1. It was not enough.
Two minutes later, Arsene Wenger exchanged the athletically adventurous Alex Oxlade-Chamberlain for the now unpopular Arshavin.
Ashburton Grove responded with a cacophony of catcalls and a loud rebuke to their manager that "You don't know what you're doing".
If Wenger's long-term aim was to protect an 18 year old already exposed to the hyperbole that envelopes young English talents ("He was sick on Sunday and he started to stretched his calf," the manager explained), it is the short-term that is imperilling Arsenal.
Full-backs are absent as they fall back away from the next Champions League.