Ronald Koeman is fast becoming a hero at Southampton after guiding the club to their highest ever Premier League finish, narrowly missing out on Champions League qualification.

The Dutch manager built himself a solid defensive unit that provided the backbone of the their success.

Fraser Forster impressed in goal, Nathaniel Clyne and Ryan Bertrand were strong in attack from the full-back role and the centre back pairing of Jose Fonte and Toby Alderwiereld provided a rock that kept quiet some of the best strikers in the league.

The Saints conceded only 33 goals last season, keeping 15 clean sheets in the process. That's a remarkable record for a team not considered amongst the best in the division, and only champions Chelsea were better defensively, conceding one less than the South East side.

Manager Koeman faces a major challenge to replicate that form this season, and he heads into the new campaign with only one of that defensive lineup fit to play; captain Jose Fonte.

Clyne and Alderwiereld have both moved on to new sides, with Clyne becoming the latest Southampton player to join Liverpool and Spurs swooping to bring in Belgian defender Alderwiereld.

As for the other two, both players are in the middle of lengthy spells on the sidelines through injury. Fraser Forster is recovering from a knee injury that ruled him out of most of the second half of the season alongside left back Bertrand, who is also out with a knee problem.

If that didn't leave the Saints short enough, centre back Florin Gardos astonishingly becomes the third bad knee injury in recent months, and he faces seven months on the sidelines.

Koeman is being forced into the market to add numbers to his defensive options, the former Barcelona player has said he wants one or two more central defenders before the start of the season.

He did managed to bring QPR centre-back Steven Cualker in on loan earlier this week, but he certainly needs further reinforcements to ensure his defensive options are up to scratch ahead of the new campaign.

Here are three targets the Dutchman should be looking at...

Virgil Van Dijk - Celtic

The Dutch centre back has become an important part of everything good at Celtic. Available for around £10million, he would be a great addition to the Saints' back line.

Van Dijk has spoken of his desire to move to a better standard of football and to impress ahead of next summer's European Championships in France.

Koeman's Dutch revolution would provide the perfect opportunity to do just that.

The Dutch defender is highly regarded by most. He's a big, strong defender with excellent technical skills, and has the intelligence to read the play and adapt his position to have the best chance of recovering the ball.

It is not just in defence where the 24-year-old shines, though. He scored 10 goals for Celtic last season, and regularly impressed at both ends of the field.

Bruno Martins Indi - FC Porto

Ronald Koeman has been a fan of Martins Indi for a long time, even failing to lure him to the Premier League last summer when the player decided to move to Porto instead.

The Dutch manager learned a lot about the defender from facing his side during his time with Feyenoord and was always impressed by the youngster's talent.

Martins Indi can play either in the centre of defence or at left back and would be a real coup for the Saints if they could lure him to St. Mary's, though Porto are likely to want to make a decent profit on the £6million they paid for him last summer.

The stocky centre-back possesses obvious strength and has been commanding at the back during his time in Holland. The player is also great in the air and, surprisingly for most defenders, has great composure, with his ball retention a favourable attribute of his game.

At 23 he is another talented defender with a very bright future ahead of him.

Hector Moreno - Espanyol

Moreno is a real talent in defence and has been drawing attention from some of Europe's top clubs for a number of seasons.

The Mexican international has become a vital part of the backline for both club and country, Moreno is Mexico's best defender and would be available for around £8million this summer.

A strong defender in the tackle, Moreno is not the big physical type of centre-back you normally get, playing with a bit more skill and technique than most. The Mexican is comfortable on the ball, having recorded one of the best passing accuracy ratios in the Espanyol squad last term.

Alongside the player's technique, his main strength is his intelligence on and off the ball. Moreno reads the game brilliantly and is known for his interceptions.

The 27-year-old would fit Southampton's philosophy perfectly and would provide a like for like replacement for the departed Alderwiereld.