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Manchester United used THREE different centre-back combinations against Southampton on Monday night.

One change was enforced when Chris Smalling went off with an injury while one was tactical when Louis van Gaal substituted Paddy McNair and moved Michael Carrick back into defence.

It means United have used 18 different combinations at centre-back in just 15 Premier League games so far this season.

Here's a look at how each one has got on so far. Marcos Rojo and Smalling is the most reliable partnership, starting four games together and averaging a goal conceded every 154 minutes.

1. Jones/Smalling/Blackett

Games: 2 | Games Started: 2 | Minutes together: 89 | Goals conceded together: 2

Phil Jones, Chris Smalling and Tyler Blackett started the first game of the season against Swansea as a back three. But that lasted just 45 minutes before Van Gaal reverted to a back four. The same three-man defence was back for the next game against Sunderland, but Smalling limped off after 44 minutes and was replaced by Michael Keane.

2. Smalling/Blackett

Games: 2 | Started: 0 | Minutes: 98 | Goals conceded: 5

Smalling and Blackett were first paired together in the second half of the opening day defeat to Swansea after Van Gaal changed from a three-man to a four-man defence. They played together again when Smalling replaced the injured Evans at Leicester as United collapsed in the final 30 minutes. United were forced to play the final seven minutes with 10 men after Blackett's red card. As a pair, they've conceded a goal every 20 minutes.

3. Jones/Blackett/Keane

Games: 1 | Starts: 0 | Minutes: 46 | Goals conceded: 0

When Smalling limped off just before half-time against Sunderland in August, Keane replaced him in the back three. It was Keane's first and only Premier League appearance of the season before joining Burnley on loan on deadline day.

4. Jones/Evans/Blackett

Games: 1 | Started: 1 | Minutes: 90 | Goals conceded: 0

Jones, Evans and Blackett have started as a back three once this season, in the goalless draw with Burnley at the end of August. However, by the end of the international break, Jones had picked up a hamstring injury playing for England and Van Gaal reverted to a back four for the next game against QPR.

5. Evans/Blackett

Games: 2 | Started: 2 | Minutes: 120 | Goals conceded: 1

Van Gaal started the first three games of the season with a three-man defence. But for the first game after deadline day - QPR's visit to Old Trafford - he started with a back four for the first time.

Evans and Blackett were Van Gaal's first pair and helped keep a clean sheet in the 4-0 win over Harry Redknapp's side. Evans and Blackett both started the next game at Leicester, but Evans was forced off with an ankle injury after 30 minutes with United leading 2-1. They went on to lose 5-3.

6. Rojo/McNair

Games: 2 | Started: 2 | Minutes: 180 | Goals conceded: 2

Of the 13 combinations Van Gaal has used this season, Rojo and McNair have played the most minutes together. With injuries to Jonny Evans, Chris Smalling and Phil Jones and Tyler Blackett suspended after his red card at Leicester, McNair was given his senior debut for the 2-1 win over West Ham in September.

McNair acquitted himself well alongside Rojo - who made his first two United appearances at left-back - and the pair were kept together for the 2-1 win over Everton a week later. McNair might have stayed in the team had it not been for a hamstring injury and Van Gaal was forced into another change.

7. Rojo/McNair/Thorpe

Games: 1 | Started: 0 | Minutes: 1 | Goals conceded: 0

United were hanging on at the end of their 2-1 win over West Ham at Old Trafford after Wayne Rooney had been sent off. And with time ticking away Van Gaal threw on Tom Thorpe to join Rojo and McNair to make sure United didn't give away a late equaliser.

8. Rojo/Jones

Games: 1 | Started: 1 | Minutes: 90 | Goals conceded: 2

After missing four games with a hamstring injury, Jones returned to start against West Brom at the Hawthorns in October. He was paired with Rojo at the back as United conceded twice in a 2-2 draw. Jones hasn't played since after developing shin splints.

9. Rojo/Smalling

Games: 4 | Started: 4 | Minutes: 309 | Goals conceded: 2

Rojo and Smalling were first paired together for the 1-1 draw with Chelsea in Ocotber. They kept their places for the trip to Manchester City, but Smalling lasted just 39 minutes before he was dismissed for two bookable offences. It meant Michael Carrick was pressed into action at the back for the first time this season.

They were reunited for the 3-0 win over Hull in November and the 2-1 win over Stoke four days later before Van Gaal reverted to a back three against Southampton.

10. Carrick/Rojo

Games: 1 | Started: 0 | Minutes: 17 | Goals conceded: 0

Carrick and Rojo played together for 17 minutes against Manchester City following Smalling's red card. The partnership was ended when Rojo dislocated his shoulder in the second half and was replaced with McNair.

11. Carrick/McNair

Games: 2 | Started: 0 | Minutes: 53 | Goals conceded:1

Another centre-back pairing thrown together by circumstance. Carrick and McNair both started the Manchester derby at the Etihad Stadium on the bench. But after Smalling's red card and Rojo's injury, they played the final 34 minutes in the centre of defence.

Blind started at the back in the next game against Palace, but Van Gaal's switch meant Carrick and McNair were back together midway through the second-half until the Northern Irishman went off in the final minute with cramp.

12. Blind/McNair

Games: 1 | Started: 1 | Minutes: 70 | Goals conceded: 0

Injuries to Jones and Evans and Smalling's suspension meant Van Gaal was forced to field a makeshift back four of Antonio Valencia, Luke Shaw, Daley Blind and McNair against Crystal Palace. United kept a clean sheet, but halfway through the second-half Van Gaal switched Blind and Carrick with the Dutchman going back into the midfield.

13. Carrick/Blind

Games: 1 | Started: 0 | Minutes: 1 | Goals conceded:0

Carrick and Blind played the final minute of the 1-0 win over Crystal Palace after McNair went off with cramp. Both picked up injuries while on international duty.

14. Smalling/McNair/Blackett

Games: 1 | Started: 1 | Minutes: 89 | Goals conceded:0

After the goalless draw at Burnley in August, Van Gaal ditched his three-man defence for the next seven games. But he brought it back for the trip to Arsenal in November. Smalling, McNair and Blackett were brought together for the first time and, despite a shaky start, didn't concede. They were broken up in the final minute when Darren Fletcher replaced Ashley Young and Blackett was switched to left wing-back.

15. Smalling/McNair/Carrick

Games: 1 | Started: 0 | Minutes: 1 | Goals conceded:1

When Fletcher came on for Young in the final minute at Arsenal, Carrick dropped back into defence next to Smalling and McNair. But he couldn't stop Olivier Giroud scoring what turned out to be a consolation.

16. Smalling/McNair/Rojo

Games: 1 | Started: 1 | Minutes: 18 | Goals conceded:0

Smalling, McNair and Rojo were chosen in a back three against Southampton at St Mary's. But Smalling lasted just 18 minutes before limping off with an injury with United 1-0 up and was replaced with Evans, who made his first appearance since September.

17. Evans/McNair/Rojo

Games: 1 | Started: 0 | Minutes: 21 | Goals conceded:1

Evans came on for the injured Smalling at St Mary's, and 15 minutes later Graziano Pelle scored Southampton's equaliser. McNair was substituted before half-time and replaced with Ander Herrera with Carrick dropped into defence alongside Evans and Rojo.

18. Evans/Carrick/Rojo

Games: 1 | Started: 0 | Minutes: 51 | Goals conceded:0

Evans, Rojo and Carrick played together for the 51 minutes against Southampton. They were thrown together with the score 1-1 and didn't let Southampton score again as United ran out 2-1 winners thanks to Robin van Persie's double.