There was one sequence midway through the first half that perhaps best encapsulated the good and the bad of Patrick Dorgu the left back.

With the game locked at 0-0, Sabah broke on the counter and there was Dorgu, as far forward as any United player, appearing totally uninterested in sprinting back. It did not go unnoticed.

But then, seconds later, Sabah were dispossessed in the middle of the park, the ball was turned over and in a flash it was ferried out to Dorgu, still high and wide, to pick his head up and deliver a dead-eye cross to the near post for Matheus Cunha to score. Cunha immediately pointed at Dorgu in recognition of the quality of the assist. Pandemonium followed in the stands.

United's second goal owed a lot to Dorgu, too.

Youri Tielemans, who was brilliant before he went off at half-time, gave it to United's 'new' left back near the halfway line and he strode with purpose before playing it out to Cunha.

Dorgu overlapped, willingly as those are the positions he loves to take up, and Sabah's back-line had to hedge having seen the damage his crosses could do. That hesitation opened up a passing lane for Tielemans to receive and spin in the box, cut it back to Bruno Fernandes and up went the roar around Old Trafford when it nestled into the bottom corner.

So by the time the players filtered off the pitch, Dorgu had the UEFA Player of the Match trophy in his hands. A huge positive, even if Dorgu the left back remains very much a work in progress.

Before getting in to the specifics of where Dorgu must improve it's important to stress it was a night of huge positives in what many dubbed a left back audition. Those behind the scenes at United feel it can become something much more permanent.

There was something of an irony to the fact that Dorgu looked the most dangerous wide player on the pitch going forward, and yet somehow also looked capable of bringing the most danger to United's door going backwards.

Where Luke Shaw, missing from this clash due to the after-effects of cramp sustained at Everton - where he was brilliant - cannot sustain the attacking output that brings a whole new dimension to this team, elevating the likes of Cunha immeasurably, Dorgu can. And yet this man-of-the-match display still showed the Dane has plenty to do bring Shaw's level of calmness at the back.

For all the good here (and there was a lot) - for a start he had more touches in the box (8) and created more chances (3) than any other player on the pitch at half-time - a willing learner like Dorgu will come away with plenty to work on.

There were multiple moments where Dorgu over-committed, where Dorgu misjudged the press and where Dorgu appeared to have little appetite to do the defensive side of the game. He looked like a winger playing left back when the odd occasion arrived to defend.

When tougher assignments come up, he won't get bailed out. Lisandro Martinez told him as much in one rather tense first half exchange after being bypassed by Kaheem Parris.

Before coming off on 70 minutes he was beaten off his spot in a turnover and pulled down his man cynically for a booking. An acute reminder with Manchester City on the horizon.

The truth is that this is as new for Dorgu as it is for Carrick and his staff.

He arrived as a left back from Lecce who made his debut at right wing-back. When Ruben Amorim reluctantly went to a back four against Newcastle United on that infamous Boxing Day, he was pushed to a right-wing role.

Darren Fletcher, who held the fort as an interim between Amorim and Carrick, platformed him as a left winger and Carrick has mostly followed suit. His crossing is a real strength and his athleticism allows him to constantly provide an outlet out wide. That much is largely undeniable.

But then United didn't get a left back this summer. They couldn't get Lewis Hall out of Newcastle United and they can't expect Shaw to play twice a week. His 'rest day' on Wednesday was evidence enough of that.

So, United need to invest in Dorgu as a left back. Between now and January, or now and the end of the season, United will be a far stronger unit with Dorgu hitting all the right notes at both ends from left back.

They have Marcus Rashford who can play left wing. They have Cunha, too. What they don't have is an abundance of left backs and that is why Dorgu must become more than just a European experiment.

Against a Sabah side that targeted Dorgu's side much more so than Noussair Mazraoui on the opposite flank, Carrick and his staff can be forgiven for asking Dorgu to stay high and wide in a bid to overload at the top end of the pitch. This was a one-sided contest for the most part.

Tougher tests will come, particularly so this weekend if Shaw is not fit enough to start against Manchester City back here at Old Trafford on Sunday afternoon. A lack of intensity in your defensive duties against Antoine Semenyo or Iliman Ndiaye is very different, with all due respect, to doing so against Parris and Co.

Some of it was effort, some of it was tactical and positional. If Carrick and his staff want Dorgu to play as high as they did here, the left centre back, in this case Martinez, needs more protection. At times the gaps were glaring.

But a 4-0 rout of a European minnow is a good time to try something new and Dorgu came through it with a trophy in his hands.

Carrick said this week that Dorgu can be 'like a new signing' and any new signing would need time to adapt.

Here was Dorgu playing left back for the first time under Carrick and playing in the Champions League for the first time, too. He needs time to learn and adapt.

The good: Dorgu helped unlock the team in attack, helping Cunha to produce his best performance of the season and create extra space for Fernandes and Tielemans to operate in. The bad: he forgot to lock the door at the other end in an up-and-done first 45.

How Dorgu kicks on from here will have a huge bearing on the left back recruitment for January and next summer.

The fundamentals are all there and team-mates are elevated in their proper positions when a left back in Dorgu's mould enters the fold. So much of playing him there makes sense.

So, is the long term answer at left back actually already in the building?

Only Dorgu can prove whether or not he's the man to succeed Shaw for more than just the visit of Sabah.