David de Gea and Rio Ferdinand led the protests after Manchester United conceded against Middlesbrough under the most controversial of circumstances.
The Red Devils saw their narrow lead in Friday night's FA Cup fourth-round clash evaporate shortly after the hour mark as Matt Crooks cancelled out Jadon Sancho's first-half opener.
The Middlesbrough midfielder tapped in from close range after Duncan Watmore dinked the ball over Dean Henderson from the other side of the penalty area.
