Spurs, who were beaten in the League Cup at Wembley last month, were on the back foot inside nine minutes after United midfielder Marouane Fellaini finished past France international Hugo Lloris.
Michael Carrick guided a header past the Spurs goalkeeper to double the home side’s advantage before United captain Wayne Rooney wrapped up three points with over 51 minutes to play at Old Trafford.
Speaking to BBC Sport following his side’s humiliating defeat, Pochettino said: “We are disappointed because our performance today was very poor and we have to analyse why.
“After we conceded the first goal it seemed that the game was finished and we never found a way to damage Manchester United. We were never in the game.”
The north London side are six points behind fourth-placed United following their defeat in Manchester on Sunday.
Spurs face Newcastle United, Southampton, Manchester City and Everton in their final nine Premier League games of the season.
Tottenham have won seven of their 14 games on the road in the English top-flight this term.
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