Ten Hag selected summer signing Christian Eriksen as a false nine to begin the match, only bringing on Ronaldo, who had only played just 45 minutes of pre-season, in the 53rd minute for Fred.
Manchester United found themselves two goals down inside 39 minutes after twice being exposed on the counter-attack which allowed Pascal Gross to score on both occasions.
Ronaldo could not help change the result as, despite Alex Mac Allister's own goal midway through the second half, the Seagulls held on for their first win at Old Trafford.
After the game Ten Hag remained firm with his choice of team selection.
"If he was available, a striker, I would have played him," he said.
"Cristiano Ronaldo was maybe for 10 days in team training and that is too short for 90 minutes so that is the reason we didn't start him."
Ten Hag insisted the home defeat to Brighton was "unnecessary" and he put it down to a lack of self-belief.
"It was unnecessary to lose this game," said the Dutchman.
"I think it was a good start and then after (the first goal) we dropped down a level, dropped down in belief and we made mistakes and the opponent punished us.
"But I think it is a good opponent, they have worked long together and when you make mistakes you get punished for that against such teams."
Asked what positives he could take from the game, Ten Hag added: "The second half, dealing with the setback.
"We dropped down in belief in the first half and that cannot happen, you always have to believe in yourself and stick together as a team.
"We came back in the second half and we had to believe on the pitch and that was good see.
"They have to deal with those setbacks and give themselves belief because they are really good players.
"When you are a well-constructed team you will play well and that is what we saw second half."