After Romelu Lukaku's 20th-minute finish had given United the lead, they struggled to perform with the same fluent authority that has inspired their fine start to the season.

That Southampton remained goalless owed much to the form of Bailly and Jones.

And asked about their performances, Mourinho said: "Very good, and also [substitute Chris] Smalling. We did for 20 minutes what the majority of the Premier League teams are doing, which is to play defensively with five at the back.

"We had chances to kill the game and score the second goal, but didn't.

"Then, Southampton were strong. I felt the manager [Mauricio Pellegrino] was enthusiastic, exciting, risking bringing on two strikers like [Manolo] Gabbiadini and [Charlie] Austin to play direct, and also in a style that isn't normally their style."

Saturday's victory follows four others in the Premier League that had been by wider margins, and Mourinho, dismissed late on for appearing to clash with fourth official Mike Jones, said: "It's better to win 5-0 than 1-0. We wanted to win also 5-0.

"But it's not always possible. We had opportunities in the first half to score the second goal and then the game is different.

"Lukaku and [Ander] Herrera, too, and if we'd done that I would have brought on [Antony] Martial and [Jesse] Lingard and go for a different result.

"But credit to Southampton and to Pellegrino."