Two first-half Pascal Gross goals set up the win for the Seagulls before United rallied in the second half, buoyed by a 68th-minute Alexis Mac Allister own goal.
Ronaldo, who has had a long-running transfer saga at Old Trafford during the off-season and didn’t join United’s pre-season tour of Thailand and Australia for family reasons, was injected into the game in the 53rd minute.
Prior to Mac Allister’s own goal, Ronaldo broke down the right flank and set up Marcus Rashford, whose routine chance was blocked by Brighton keeper Robert Sanchez.
Ten Hag explained why Ronaldo didn’t start, when he said: “It takes time.
“We cannot force it and one week in training, a little bit more now, he has to do more to get fit and this game will help him.
“The 35 to 40 minutes he had now and now we have one week and he will be better next week.”
The Dutchman admitted his disappointment at the home defeat but was pleased with the response from a second-half tactical switch with Christian Eriksen bossing play deeper.
“Disappointed, it is clear,” he said. “We of course had other expectations but that happens and this is also a process, it takes time and we have to work really hard. This result was unnecessary.
“It was clear to see that in the second half we were better in the midfield, with Christian Eriksen back down and Cristiano Ronaldo further up.
“Then we create and Rashford had two really good chances and it’s a pity we didn’t score to make it 2-2.”