When the Bundesliga season got underway, Borussia Dortmund had ambitions of providing much stronger competition to champions Bayern Munich than their 19-point deficit last time around. For the second summer running since winning back-to-back titles, Jürgen Klopp’s men had lost one of their key players to Bayern, with Robert Lewandowski following Mario Gotze’s path. But Dortmund had reinforced to seemingly make their squad, if not their team, stronger.  

No one could have predicted what has followed. A scoreless result at Bayer Leverkusen in the first match after the Bundesliga’s winter break left the 2013 Champions League runners-up at the bottom of the table. Their form has been much better in Europe, where they topped their Champions League group to reach the last 16.

Barring an astounding triumph in Berlin in May, Dortmund will miss out on the hugely lucrative and glamorous competition next season. The priority now is simply ensuring that they’ll be in Germany's top league. Even should they accomplish that goal, the club is likely to find it tougher than ever to hang onto their best players, with a number of Europe’s leading teams circling amid uncertainty over the future of Klopp and of the remarkable project he has led in recent seasons.

Here are the details on the players likely to be targeted.

Marco Reus
Reus chose Dortmund over Bayern Munich when leaving Borussia Monchengladbach in 2012 and has proved to be a superb signing. Operating behind a striker or cutting in from the left, the 25-year-old was named the Bundesliga’s Player of the Year last season after scoring 16 goals and adding 13 assists. His progress in the past few months has been hampered by injury, forcing him to miss Germany’s victorious World Cup before suffering further ankle problems this season. But that injury record is unlikely to deter bids this summer, given that a release clause of 25 million euros (£19 million) will become active. For a player of his ability, the price represents remarkable value for money.

Teams interested: Bayern Munich have long been seen as favorites to land Reus in the summer, although recent reports in Germany suggest they may have eased their interest. Chelsea may also be out of the running after signing Juan Cuadrado on Monday. Although Real Madrid are even more well-stacked for attacking talent, there have been stories that the European champions are looking at Reus as a replacement for Gareth Bale. That level of competition could well mean that Arsenal, also credited with an interest despite the number of players they already have in his position, may struggle to lure Reus to the Emirates stadium.

Value: £19 million

Mats Hummels
A World Cup winner with Germany last summer, Hummels is one of the most coveted center-backs in the world. Exceptional on the ball and a strong leader, it is easy to see why that’s the case, although his lack of pace can see him exposed on occasions. For those keen to secure his services, a major obstacle has been Hummels’ insistence that he is happy at a club he joined after failing to make the breakthrough at Bayern Munich. While he has stated that a lack of Champions League play wouldn’t change that stance, he is likely to be presented with some tempting offers in the summer.

Teams interested: Manchester United have been strongly linked with a transfer since last summer and are believed to have made attempts to sign him in the January transfer window. While that was always unlikely to happen in mid-season, it could be a different story in the summer. In recent windows, the Premier League club have shown an ability and willingness to spend over the odds to get the players they want and they could make a bid that’s hard to refuse for the 26-year-old. Arsenal may also register their interest, but they could find themselves outmatched financially,

Potential value: £35 million

İlkay Gündoğan
Gündoğan was an integral part of the Dortmund side that reached the final of the Champions League two years ago, setting the tempo with his passing and his pressing off the ball. But he has hardly played since having missed more than a year with a back injury before returning last October. He is now showing signs of getting back to his former level, which is likely to ensure significant interest in him this summer. The 24-year-old signed a new contract last year, but it only ties him to the club until the summer of 2016, meaning that it is highly likely he will either sign a longer term deal or be sold at the end of this season.

Teams interested: Arsenal have been strongly linked and Gündoğan would be a good fit at the base of their midfield. The same is true, though, for Manchester United, who could now turn their attention more firmly toward the Germany international following another serious injury suffered by their main midfield target Kevin Strootman. Before his injury, Real Madrid were also keen, but their recent signings of Toni Kroos and Lucas Silva mean a bid is now unlikely.

Potential value: £20 million

Sven Bender
More of a defensively focused midfielder than Gundogan, Bender has been an unheralded part of Dortmund’s success and a regular member of Germany squads for several seasons. The 25-year-old, brother of Bayer Leverkusen’s Lars, has also endured more than his fair share of injuries in recent times and missed last year’s World Cup. He is currently on the sidelines again after undergoing surgery on his knee last month.

Teams interested: Arsenal have reportedly twice tried to sign Lars Bender, but have also been linked with Sven of late. Last summer, the player revealed that the Premier League would be his destination of choice if and when he leaves the Westfalenstadion. In the close season he will also enter the final two years of his contract.

Potential value: £15 million