Pochettino said: "We didn't start the match well and maybe that can be expected because of the last couple of days.
"We couldn't match the energy of Manchester United but they had an extra day to recover. We missed energy and the possibility to change the game. That was a problem and it happens sometimes. That is football.
"Full credit to Manchester United. They deserved the victory and were better than us but we were close. We need to be focussing on the next game."
He added: "We need to be more clinical in the places like Old Trafford. We conceded the second goal and it was more difficult. We need to make better decisions. I think they pushed us to play in our half in the first half but then I think we were better and played a little bit better. It's hard with all of the emotion out there."