Fulham won at Old Trafford for the first time in more than two decades with a 2-1 victory in the Premier League at the weekend. The defeat left United in sixth place, eight points behind fourth-placed Aston Villa.
United, who suffered a 2-1 loss to rivals Manchester City in the FA Cup final at Wembley last season, visit Nottingham Forest's City Ground on Wednesday night.
"We were unbeaten in January and February and we lost one game. It was a poor performance and defeat, we are aware," Ten Hag told reporters.
"We want to stay in every competition. We have to win tomorrow. It doesn't change our approach - that is for every game.
"I look at today, we work on the team development and try to win every game. We look forward to the longer-term."
The Dutchman urged patience with young players like Kobbie Mainoo, Alejandro Garnacho and Omari Forson.
"They (youngsters) need time but they also need to know that at the top level you don't get much time," Ten Hag said. "That is the paradox. I will give them time because I believe in these players."
Ten Hag also backed Brazilian winger Antony, who has recently been linked with a move away from United, to fight his way back into the starting line-up after poor form. Ten Hag coached Antony during their time at Ajax.
"I know from the past he is unstoppable. He's one of the quickest players in the first 10 yards. When he plays this game, he will perform," he said.
"I'm confident he will do it in the future... He is resilient and he is a character. He will fight back."
Ten Hag said midfielder Casemiro was available for the Forest clash.
"He's available. He was bleeding so he had to come off (against Fulham)," Ten Hag said.
