On the Dutch manager’s Premier League debut, a brace from Pascal Gross secured a famous Old Trafford win for the Seagulls.
Ronaldo, who spent the early weeks of the summer absent from training due to personal reasons - while also attempting to force a move out of the club - began the game on the bench before his introduction on 53 minutes of the 2-1 defeat.
“It takes time, you cannot force it,” he told Sky Sports. “He is one week in training. He has to do more to get fit. This game will help and he will be better next week.”
The former Ajax boss added: “It was clear to see in the second half we were better in the midfield, Christian Eriksen down [in midfield and Cristiano Ronaldo up [front].
“We created and had really good chances. It is a pity we didn't score them.”
United head to Brentford next Saturday after a poor start to Ten Hag’s reign as manager.
He noted: “Of course it is definitely a setback, a real disappointment. We have to deal with it.
“I knew from the start it wouldn't be easy. It is a process, it takes time - but you don't have time. You have to win games. We should have done better. We have to analyse it and take the lessons.”