Last season was a coming of age one for Rafael da Silva, the right-back was superb on a consistent basis on United's way to winning the league, with memorable goals against QPR and Liverpool.
This campaign has been a frustrating one so far for the Brazilian. After a hamstring injury delayed his start to the season, he returned to the first team, and was then taken off at half-time against Fulham with another suspected knock.
Manager David Moyes has confirmed the defender has been left at home for their Champions League match with Real Sociedad, suffering ankle ligament damage.
The Telegraph report that Rafael faces a 'battle to be fit' for Sunday's 4.10pm kick-off against Arsenal, and is officially 'doubtful.'
They add: "United's medical staff have yet to rule Rafael out of the Arsenal game, but he is understood to be 'touch and go' for the clash with Arsene Wenger's team."
Potentially it is a very big boost for Arsenal in the pivotal Premier League match, with United lacking a proven specialist back-up.
Phil Jones covered for Michael Carrick in midfield at the weekend, and could be needed there again, with Chris Smalling more likely to deputise - as he ably did for England at Wembley against Poland last month.
It is however no simple injury boost for Arsenal, with their own left-back Kieran Gibbs, who would have clashed with Rafael on the flank, also battling his own fitness issues with a calf strain.
Arsenal also do not possess a top class left-winger who could cause United problems. There certainly won't be a repeat of the mismatch which saw Antonio Valencia bamboozle Andre Santos at Old Trafford last season.
Arsene Wenger played Santi Cazorla out on the left against Liverpool, and likely will again, but the Spaniard is not a traditional winger, and is most dangerous when he cuts inside, as evidenced by his goal against Liverpool.
His best hope of success is to pull Smalling out of position and inside, and hope Arsenal's left-back, whether it be Gibbs or Monreal can take advantage of the space by double teaming the defender.
Manchester United fans will surely be crossing their fingers Rafael is fit to return, but if not the onus will switch to the Gunners to see if they can expose the fact the champions are without their first choice right-back.
Can Arsenal use Rafael's potential absence to turn the game? What do you think?