Ruben Amorim said Manchester United are facing a "long journey" back to the top after Nottingham Forest recorded their first win at Old Trafford for 30 years.
Forest won 3-2 on Saturday to condemn Amorim to back-to-back top-flight defeats for the first time since August 2022 when he was in charge of Sporting CP.
The result leaves United 13th in the table with just 19 points -- their lowest tally after 15 games since the 1986-87 season.
"We already knew [it would be tough]," Amorim said.
"It will be a long journey but we want to win because this is a massive club. You feel it, when you lose one game it's really hard for everybody.
"I can understand that. I can feel it in the stadium after the first goal. We understand the context but we have to keep going in the same way, doing the same things. The same words I have here with [the 4-0 win over] Everton, I have today. Focus on the performance. We need to improve in a lot of aspects of the game and continue to do the same things tomorrow in training."
United were undone by a series of defensive mistakes which allowed Forest to add victory at Old Trafford to their 1-0 win over Liverpool at Anfield in September.
Nikola Milenković scored with a powerful header after just 90 seconds when Lisandro Martínez failed to jump for a corner.
A misplaced pass from Bruno Fernandes and a howler from André Onana allowed Morgan Gibbs-White to score the second and there were a catalogue of errors from Onana, Martínez and Matthijs de Ligt for the third scored by Chirs Wood.
"I had this and worse in Sporting in the beginning," Amorim said.
"The feeling for me is the same but for the world it's completely different. You know Sporting in Portugal but Manchester [United] has a lot of attention but for me it's the same feeling.
"I had this period at Sporting and if you are a little experienced in football this happens with a lot of clubs and we have to manage to continue to do the same things and improve the team because this will turn around. We need time and to continue to work in the same way every day."