4. Dries Mertens (Napoli)
Napoli’s Dries Mertens is one of the in-form players in world football right now. With 35 league goals in his last 33 matches, Europe’s top clubs have all taken an interest in the Belgian. Mertens has a ÂŁ25million release clause in his contract, meaning that someone is most likely going to get a bargain if he opts to leave the San Paolo. Manchester United have shown interest, and the 30 year-old would surely be tempted to join up with international teammate Romelu Lukaku at Old Trafford.