Coquelin has fallen behind Aaron Ramsey, Granit Xhaka and Jack Wilshere in the pecking order, while West Ham were also said to have been interested in his signing.
He has though struggled to secure playing time under boss Arsene Wenger so far this season and has accumulated only 155 minutes of football in the Premier League this term. "He had an opportunity and I let him go", Wenger told reporters after the Chelsea match.
'I nearly regret not having come before.
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Coquelin joined the club in July 2008 after impressing on a trial from home nation club Stade Lavallois, and managed to establish himself in the first team after a series of loan spells with Lorient, Freiburg and Charlton Athletic respectively.
Coquelin becomes the club's second signing of the January transfer window, and he said that speaking with now former Arsenal teammate Santi Cazorla played a role in his decision to transfer to the Spanish side.
The former France Under-21 global, who has yet to make a senior appearance for his country, says he spoke to several of his Arsenal colleagues - including Spain worldwide Santi Cazorla - before committing to a move to LaLiga. They were very good. "Obviously, I am here to help the team and to help them hit their targets".