Robin van Persie left the Liberty Stadium on crutches as Louis van Gaal branded Manchester United’s defeat at Swansea as “amazing”.

Dutch striker Van Persie was injured late on in the 2-1 defeat at Swansea as United slipped to fourth in the Barclays Premier League following Arsenal's win at Crystal Palace.

United took a first-half lead through Ander Herrera but Ki Sung-yueng equalised within two minutes and Bafetimbi Gomis gave Swansea victory when he deflected Jonjo Shelvey’s 73rd minute strike past David de Gea.

Gomis’ goal secured Swansea’s first-ever league double over United after the Welsh club’s 2-1 win at Old Trafford on the opening day of the season in August, but Van Gaal felt what was only his side’s second defeat in 20 games in all competitions was undeserved on the balance of play.

Official statistics on the Barclays Premier League website showed United had 12 shots compared to Swansea’s eight, but Van Gaal’s men managed only three on target compared to the home team’s six.

“It is always frustrating when you lose and you are the dominating team, your disappointment is much bigger because of that,” United manager Van Gaal said.

“In the first half it was an equal game as I counted as many chances for Swansea as we had.

“That was not a big difference, but in the second half it was a big difference.

“Swansea had a chance in the first minute of the second half and after that we dominated the game. Every two minutes we had a chance but we were not effective.”

Van Gaal also bemoaned the nature of Swansea’s winner when Gomis unknowingly got in the way of Shelvey’s powerful effort.

“They had one counter, the shot deviated, maybe also in an offside position from Gomis, and it was a goal,” said Van Gaal.

“Nevertheless we dominated the game even with 10 men when Robin van Persie could not run any more after his injury and I had three substitutes on the field.

“It is amazing we have lost and we have lost only to Southampton in 19 games before this, but you have to pick yourself up and continue.”

Swansea's win was only their second in eight league attempts but manager Garry Monk contradicted Van Gaal by insisting his side deserved all three points.

"We had to ride some pressure second half but we're playing Manchester United with the squad and players they have," Monk said.

“They’re always going to create but I think they had only one shot on goal second half.

“I thought the first half was quite even, felt we played well and responded how you’d want your team to respond after going behind.

“We were pushing and we had a couple of chances cleared off the line, so we were more than competing.

“Through the difficult moments in the second half we stuck together and I thought we deserved the win.”

Monk admitted he took huge satisfaction from Swansea’s first-ever league double over United.

He said: “We’ve created history by taking six points off United and we’ve performed extremely well to do it.

“To do the double over United for the first time in our history, everyone connected with the club should be extremely proud of that.”