Erik ten Hag described Manchester United's dire opening-day defeat to Brighton and Hove Albion as "a real disappointment", as he backed wantaway striker Cristiano Ronaldo to improve following his second-half cameo.

Ten Hag's United tenure got off to a nightmare start as Brighton claimed their first ever win at Old Trafford at the 15th attempt, with Pascal Gross scoring a first-half brace before an Alexis Mac Allister own goal set up a tense finish.

But the Seagulls held on for a memorable victory as Ten Hag became the first United boss – including caretakers – to lose his first Premier League match at the helm since Louis van Gaal did so against Swansea City in August 2014.

With Ronaldo starting on the bench, United faced 12 shots and managed just five of their own in the first half of the contest as Brighton produced a polished display, with this just the third time one of Ten Hag's teams have conceded 12 attempts before the break in 229 top-flight matches.

Speaking to Sky Sports after the demoralising defeat, Ten Hag bemoaned United's lack of defensive organisation and called for them to improve quickly.

"Of course, it's definitely a setback, a real disappointment, and we have to deal with it," he said.

"I knew from the start it would not be easy, it's a process, it takes time but you don't have time. You have to win games and we should have done better.

"I analysed it and it's unnecessary, we gave two easy balls away, and in think in our organisation there was a big mistake, but that happens, we have to take the lessons.

"I'm not satisfied, I'm totally not happy because we lost and it was unnecessary, we should have done things better. I think also with those players, we could have done better."

United's shot count did improve with 12 attempts in the second half, as Ronaldo's 53rd-minute introduction allowed debutant Christian Eriksen to drop into a deeper position after he started in a false nine role.

Ronaldo's future at Old Trafford remains uncertain after he reportedly professed his desire to leave in search of Champions League football, but Ten Hag was happy with his contribution from the bench and is backing the 37-year-old to improve as he gains match fitness.

"It's clear to see that in the second half we were better in midfield, with Christian Eriksen down and then Cristiano Ronaldo up," he added.

"Then we created. I think [Marcus] Rashford had two very good chances, and it's a pity we didn't score for 2-2.

"It takes time, you cannot force it, and he [Ronaldo] is now one week in training. He has to do more to get fit, this game will help him, the 35-40 minutes he had now, and he will be better next week."

United's next Premier League outing sees them travel to Brentford next Saturday.