David Moyes wanted a midfielder and a left-back this summer.
But he failed to land a full-back, missing out with bids for both Fabio Coentrao and Leighton Baines.
With Patrice Evra turning 33 before the season is out and Alex Buttner failing to prove he's a viable long-term replacement, Moyes is expected to go back into the transfer market, either in January or next summer.
January moves are complicated by the Champions League because Moyes is unlikely to want to sign a cup-tied player if United are still in the competition.
But here's a list of left-backs for Moyes to potentially pursue in January.
They're listed in order of the likelihood of arriving at Old Trafford in the next transfer window, with consideration given to the player's desire to leave and the club's willingness to sell.
The 25-year-old German international would be a wonderful signing for Moyes and United.
But it's unlikely to happen.
Schmelzer has made himself the first-choice left-back at the club he grew up at and only signed a new four-year contract in January.
But Moyes showed ambition to target the biggest names in the summer with reported moves for Cesc Fabregas, Daniele De Rossi and Sami Khedira.
But unfortunately for him, any bid for Schmelzer, especially in January, is likely to go the end way.
He's only been capped four times by Brazil, having been kept out of the team by Real Madrid's Marcelo.
But he's regular at Atletico, who've made a phenomenal start to the season, making a perfect start in La Liga and winning both their Champions League games against Zenit and Porto.
There aren't many better attacking left-backs in the world, but it's debatable whether he'd want to leave the Spanish capital.
The 18-year-old is one of the most exciting English prospects in the Premier League.
He signed a five-year contract at Southampton in July, but it won't be long before the big clubs come calling.
He can get forward, bursting into the penalty area to win a penalty for Rickie Lambert to convert in the opening day in over West Brom.
And he can defend as well, forming part of a Southampton defence that have conceded just twice in the Premier League this season.
Moyes has shown he is willing to put his confidence in young players by blooding Adnan Januzaj this season.
And signing a left-back who could be at the club for the next 10 years may well appeal.
But Southampton have tied him down to a long-term contract and would likely demand an "English Premium" fee for a player they've nurtured since the age of eight.
If the summer transfer window had been open for another hour, Fabio Coentrao would already be a United player.
United, Real Madrid and the Portugal international left-back were all agreed on a season-long loan deal, but the paperwork didn't go through in time.
He hasn't been frozen out by Carlo Ancelotti at the Bernabeu—he's started three times in La Liga this season—but the 25-year-old was left out for both Real Madrid's Champions League games.
Moyes and Coentrao were clearly keen on the move in the summer, and the deal could be resurrected in January.
Moyes' No.1 left-back target.
The United manager knows all about the 28-year-old's qualities from their time together at Everton.
His delivery from open play and set pieces is world-class, and his ability from dead balls around the penalty area make him a genuine goal threat.
Everton drove a hard bargain over Marouane Fellaini in the summer, and they'll do the same if United make another move for Baines in January.
But Moyes perused him relentlessly in the summer, only dropping his interest at the last minute to make sure he landed Fellaini.
However, if Everton continue their fine early-season form and are challenging for a Champions League place come January, Roberto Martinez is going want to keep his squad together, not sell one of his best players.