First-half goals from Romelu Lukaku and Chris Smalling secured a second successive victory for the Red Devils in the Premier League, although they failed to keep a clean sheet.
We are in a hard group, starting tomorrow with a hard team.
Speaking to Sky Sports, Mourinho said: "In this moment I'm completely in the dark".
A stunning strike is usually a good enough reason for wild celebrations, but Smalling's exquisite effort in the 37th minute from a set-piece made it all the more sweeter for the United manager, Manchester Evening News reported.
United, who have the option to extend his current contract by another season, have opened talks over a new deal with 12 months to left.
The England defender scored what proved to be a crucial second goal in a 2-1 win at Watford on Saturday evening, following on from a 2-0 victory away to Burnley before the worldwide break.
Nemanja Matic admitted to being "surprised" after the game at Watford that Smalling did not go to the World Cup after playing almost 50 games for United last season but the centre-back insists his focus now is only on his club career.
Mourinho opted for power rather than guile at Vicarage Road with a physically imposing midfield trio of Paul Pogba, Fellaini and Nemanja Matic, who was later sent off for a second yellow card. "But Marouane is giving us more than that - he's giving us that but he's also giving us quality and simplicity", he said.
De Dea, who has been the club's Player of the Year in four of the last five seasons, is widely regarded as the best goalkeeper in the world and wants a contract that reflects that. He was handed a spot with Mourinho's first team this summer.
"We made defensive mistakes against Brighton and against Tottenham. I am very happy with something that represents for me the spirit of the team, which is Lukaku's sliding tackle after 50 metres running back to help the team", he told BT Sport. If I watch Mr. Raiola on the screen saying the player wants to leave, and that he is organising a way for him to try to leave or so on, then I will believe.