Zlatan Ibrahimovic and Tyrone Mings have a 6pm deadline on Tuesday to respond to the Football Association charges for alleged violent conduct they were served on Monday.
The duo were charged by the FA after clashing at Old Trafford on Saturday.
Manchester United's Ibrahimovic caught Mings with an elbow shortly after the Bournemouth defender landed on the back of his head during a 1-1 Premier League draw.
Mings faces an extended suspension if the accusation against him is upheld, with the FA claiming the standard three-match ban for violent conduct would be "clearly insufficient".
Under FA disciplinary rules, any incident not seen by the match officials is reviewed by three ex-referees, who must all agree a red-card offence has been committed before a charge is issued.
Former Premier League official Dermot Gallagher told Sky Sports News HQ on Monday Ibrahimovic's elbow "ticks every box for a red card" and a three-match ban.
That sanction would see United's top scorer miss Monday's FA Cup quarter-final against Chelsea, as well as league meetings with Middlesbrough and West Brom.
Bournemouth's Andrew Surman was sent off in the aftermath of the 44th-minute incidents, but the visitors held out for a 1-1 draw as Ibrahimovic missed a late penalty.