At the higher end of the price spectrum is Napoli’s Kalidou Koulibaly. The Senegalese defender has impressive power and mobility that he uses to great effect in Napoli’s high line. Koulibaly is a highly valued player in Europe who has been watched by many top clubs, who will have no doubt been impressed not just with his defending, but also his ball-playing ability. The 26-year-old is confident coming out of the back with the ball, and can play accurate passes into the midfield. Such a skill would be very useful in a back three, when the side centre backs can occasionally move up the pitch into midfield areas.

In terms of size and style, Monaco’s Polish defender Kamil Glik is the closest you’ll find to Mertesacker. Glik stands at an impressive 190 centre metres, and relies on his intelligence and concentration to make up for his lack of mobility. At 29, he’s a little older than you’d prefer, but he boasts plenty of experience, having amassed 54 caps for Poland and over 300 club appearances. He’s also a significant threat on set-pieces, with eight goals for Monaco last season, and 25 throughout his career.

A younger option for Arsenal is Sevilla’s 22-year-old French defender Clement Lenglet. He joined Sevilla from Nancy last January, and has since proven himself to be a promising defender in the heart of Jorge Sampaoli’s unorthodox defensive tactics. He’s continued as a regular under new coach Eduardo Berizzo, and is important to the side due to his ball-playing ability. As a left footed defender, he’d also add something different to the Arsenal back-line.

The 22-year-old Slovakian has enjoyed a rapid rise in football, having gone from MSK Zilina to Sampdoria and now Inter Milan in just two years. Skriniar is a regular in an Inter Milan side that have conceded just nine goals all season. Style-wise, he’s closer to Laurent Koscielny than he is to Mertesacker, in that he’s willing to take more risks to win the ball back. Still, few defenders are proving to be more promising than he is at the moment, and Arsenal would be wise to keep an eye on his development.

The Uruguayan could be a bargain, as his contract expires in the summer. He broke into the Atletico Madrid first team at a young age, and although he’s struggled to hold down a regular spot, he still shows plenty of promise. He’s tall, aggressive and shows a lot of heart and passion in every game he plays. There’s a raw quality to him, but one worth refining. With over 30 caps for Uruguay, he also plenty of international experience.