Bruno Fernandes is “impossible to replace”, says Ruben Amorim, but others must lead.
Fernandes suffered a soft-tissue injury against Aston Villa and will miss several matches.
Manchester United also lack Kobbie Mainoo, who has a calf problem.
Amorim said the absences offer a chance for new leaders to emerge.
Fernandes usually organises set pieces and on-field structure.
Amorim wants players like Lisandro Martínez and Luke Shaw to assume responsibility.
He confirmed neither Fernandes nor Mainoo will face Newcastle United on Boxing Day.
Teenager Jack Fletcher is among the options to fill midfield gaps.
