The young defender had been brought on for Luke Shaw after 75 minutes but had to depart after 89 minutes following a clash of heads with Saints full-back Kyle Walker-Peters.
In his absence the hosts, who had gone ahead through Marcus Rashford and Anthony Martial before Stuart Armstrong halved the deficit, saw Michael Obafemi register a 96th-minute leveller to dent their Champions League hopes.
Manchester United defender Brandon Williams departs the pitch with a nasty cut on his eye
Young full-back is treated on the pitch by medics wearing full personal protective equipment
Williams was helped off the pitch and left his side to play the rest of the game with 10 men
After the game, Ole Gunnar Solskjaer said: ‘He’s got a cut above his eye. I haven’t been to see him yet.’
Williams was treated on the pitch by a team of physios wearing full personal protective equipment before being helped off the field.
Having used all five substitutions the hosts had to defend with 10 men, while five minutes of injury time were added on due to the delay.
Williams had only entered the action on 75 minutes as a replacement for left-back Luke Shaw
Sithout Williams on the pitch, Southampton were able to force an equaliser in the 96th minute
While a victory would have seen Manchester United leapfrog Leicester and Chelsea – who both lost at the weekend – into third, instead the club remain fifth.
They are level on points with Leicester with inferior goal difference and one point behind Chelsea with three games to play.