Manchester United could not "cope" with Swansea City's tactical switch which inspired United's first defeat of the season, according to Garry Monk.
The manager's alteration to a midfield diamond prompted Swansea's second-half comeback, which earned them a third straight league win against United.
"It's great when that happens. It was the right thing to do," Monk said after the 2-1 win at Liberty Stadium.
"It freed us up and made us much more of a threat, which was important."
Monk, who has won all three of his matches in charge against United, added: "At that point they didn't quite cope with that change and they didn't react to it well, which gave us that period where we scored the two goals."
Having seen his side fall behind to Juan Mata's goal early in the second half, Monk replaced winger Wayne Routledge with central midfielder Ki Sung-Yueng after 58 minutes.
That gave fellow midfielder Gylfi Sigurdsson licence to roam and allowed Andre Ayew to move up front.
The switch paid dividends after just four minutes as Sigurdsson crossed for Ayew to head in Swansea's equaliser.
The hosts were ahead four minutes later, as Ayew's sublime through ball set up Bafetimbi Gomis to finish beyond Sergio Romero and preserve Swansea's unbeaten start to the season.
United manager Louis van Gaal claimed his side were "dominant" for 85 minutes, and that they lost because of a five-minute spell in which they were unable to adapt to Monk's tactical alteration.
The Swansea boss, however, disputed that reading of the game: "That's a long time, 85 minutes.
"In the first half you have to give them credit. We were playing against a very good side.
"I thought they dominated the ball but didn't create much. They made it hard for us.
"But in the second half I thought we did very well. I thought we were well in the game.
"We've come away winners and that's the most important bit. I don't think anyone can really grumble with that result."
Meanwhile, Monk has played down fitness concerns about Swansea winger Jefferson Montero.
The Ecuadorian missed the win against United with a thigh injury, but Monk does not expect Montero to be ruled out for long.
"We had a scan and there was nothing clinical, so that's good news," he added.
"We'll re-scan him just to make sure but it was just soreness and tightness.
"Jeff's a very explosive player so you've got to be careful in situations like that."