Pochettino's men headed to Old Trafford on Sunday searching for a win that would have pulled them level on points with their fourth-placed hosts in the Premier League.
But Spurs were blown away as Marouane Fellaini, Michael Carrick and Wayne Rooney scored amid a poor first half for the visitors.
"The result is a consequence of our poor performance," a clearly frustrated Pochettino told his post-match media conference.
"From the beginning we started the game very slow - physically and mentally - and we didn't understand why.
"I think we were ready to compete but it is true that from the beginning it was difficult.
"Our first half was poor and at 3-0 it is difficult to come back into the game."
United's rampant form early on in the contest prompted Pochettino to introduce Mousa Dembele in place of Andros Townsend and reshape his over-run midfield after 31 minutes.
Top scorer Harry Kane was also effectively shackled throughout, but the Tottenham boss believed individual and collective errors were to blame for the defeat as opposed to any tactical failings.
"When you concede and make a lot of mistakes in football, you play in the manner that we played, it is difficult to win the game," he said.
"If you play with three or four strikers; if you play with five midfielders; if you play with three, four or five defensive players it is the same - it’s about the individual and the collective.
"We were very poor. That is why we lost the game."
Despite the damaging loss, Pochettino remains confident that Tottenham can stay in the hunt for a coveted UEFA Champions League qualification spot.
"We are in a position where all is possible but it is true that it is very difficult because we need to fight with the big, big sides," he added.
"I am very disappointed for our performance because we did not deliver it how we do when we play in a normal way.
"I don't remember when the last time was that we had a similar performance and that is why we are all disappointed."
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