On 20 March 2015, Martínez joined Championship club Rotherham United on an emergency loan until the end of the season. He made his debut the following day against South Yorkshire rivals Sheffield Wednesday, conceding two added-time goals in a 3–2 home defeat. On 2 August, Martínez was an unused substitute as new signing Petr Čech played for Arsenal in the 2015 FA Community Shield, a 1–0 win over rivals Chelsea. On 11 August, he joined Championship side Wolverhampton Wanderers on a season-long loan. After making 15 appearances in the opening months of the campaign, he suffered a thigh injury that kept him out of contention for several months and was unable to regain his starting place afterwards. He spent the 2016–17 season at his parent club, for whom he played 5 games during the campaign, including a Premier League appearance against West Ham.