United's rapidly lengthening injury list could not have come at a worse time as the Reds face a crucial period in the season.
Phil Jones and Chris Smalling are out with unspecified 'long term' injuries and Paul Pogba is absent with a hamstring problem.
Now Juan Mata joins that trio on the sidelines and looks set to miss the rest of the season after he underwent a groin operation.
The news was confirmed by United on Friday and comes as a bitter blow ahead of the imminent run in.
It is unclear how the Spanish playmaker sustained the injury.
United have placed no timescale on his return but the usual recovery time for his injury is six to eight weeks - meaning his campaign looks likely to be over.
Mata has been hugely influential as the Reds creator-in-chief this season and is the only player, excluding Zlatan, to reach double figures in his goal tally.
Mata is widely regarded as the nicest chap in football but, more importantly, his reputation as the ultimate "big game" man has elevated him to a United Player of the Season contender.
He has a happy knack of making the vital difference when it matters and, with every game crucial, his creativity and consistency will be hugely missed.
Ander Herrera and Zlatan Ibrahimovic complete their bans against West Brom, bringing United's list of injured or unavailable men up to six.
Mata's injury means Henrikh Mkhitaryan is likely to come back in on the right having been rested in the win over Middlesbrough.
Marcus Rashford will again lead the line on his own, with Jesse Lingard - man of the match at Boro - and fit again duo Anthony Martial and Wayne Rooney vying for the other attacking positions.
Ashley Young was also impressive on Teesside and could keep his place if Mourinho maintains the three-man defensive system he's relied upon of late.
At the back, the injuries to Smalling and Jones leave Marcos Rojo and Eric Bailly as the club's only fit senior centre backs - although Daley Blind could play there if necessary.
Every cloud has a silver lining, however, and its in emergencies such as this that United's reputation for blooding young talent comes into its own.
Tim Fosu-Mensah could be handed a rare chance to impress (he's made only four appearances all season), with fellow Academy graduates Axel Tuanzebe and Matty Willock also drafted into Saturday's squad to fill the void.