Mourinho's men cruised to victory thanks to goals from Zlatan Ibrahimovic, Henrikh Mkhitaryan and Marcus Rashford.
The result leaves the Black Cars 10 points off safety in the Premier League table.
And Mourinho believes Ibrahimovic's goal killed Sunderland's hopes of getting back into the match.
Manchester United player ratings against Sunderland
Sun, April 9, 2017
Express Sport rates how the Manchester United players performed during their Premier League clash with Sunderland
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Which Manchester United players impressed during their clash with Sunderland?
Manchester United manager Jose Mourinho
He told MUTV: "When we scored the goal, we played against a sad team.
"A team that is in that position is a sad team.
"And when you score before them, normally in these circumstances, you kill the match."
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