Chelsea & Man Utd stars among 12 players clubs MUST keep hold of this summer, plus Raheem Sterling

7) Mats Hummels (Borussia Dortmund)

A world class central defender, Borussia Dortmund are in big enough trouble already and must not lose Mats Hummels to Manchester United.

8) Marco Reus (Borussia Dortmund)

In another possible blow for Dortmund, Marco Reus is being linked with a host of big names and would be nearly impossible for struggling Dortmund to replace.