Martínez memulai debutnya bersama wikas sebagai pemain profesional saat dipinjamkan ke [[Oxford United F.C.|Oxford United]] saat melawan [[Port Vale F.C.|Port Vale]] di [[Football League Two|League Two]] pada 5 Mei 2012;<ref name="afc1">{{cite web|url=http://www.arsenal.com/news/reserves-news/martinez-joins-oxford-on-emergency-loan|title=Martinez joins Oxford on emergency loan|publisher=Arsenal FC|date=4 May 2012|accessdate=7 May 2012|archive-date=2012-05-07|archive-url=https://web.archive.org/web/20120507040156/http://www.arsenal.com/news/reserves-news/martinez-joins-oxford-on-emergency-loan|dead-url=yes}}</ref>. [[Port Vale F.C.|Port Vale]] menang 3-0.<ref name="oufc">{{cite web|url=http://www.bbc.co.uk/sport/0/football/17878372|title=Port Vale 3 Oxford Utd 0|publisher=BBC Sport|date=5 May 2012|accessdate=7 May 2012}}</ref><ref name="afc2">{{cite web|url=http://www.arsenal.com/news/reserves-news/43298/loan-watch-|title=Loan Watch: Wellington scores for Alcoyano|publisher=Arsenal FC|accessdate=7 May 2012|archive-date=2012-05-10|archive-url=https://web.archive.org/web/20120510060549/http://www.arsenal.com/news/reserves-news/43298/loan-watch-|dead-url=yes}}</ref>
Dia masuk dalam daftar pemain cadangan [[Arsenal F.C.|Arsenal]] saat laga tandang melawan [[Stoke City F.C.|Stoke City]] pada 26 Agustus 2012, dan melawan [[Liverpool F.C.|Liverpool]] pada 2 September 2012 pada saat [[Wojciech Szczęsny]] dan [[Łukasz Fabiański]] mengalami cedera dan pada saat melawan [[Southampton]] pada 1 Januari 2013.
