The 26-year-old Republic of Ireland international is in the final year of his contract and wants regular first-team football.
Meanwhile, Manchester City midfielder Mateo Kovacic will miss the Club World Cup after undergoing Achilles surgery.
He returned to face Bournemouth in the Premier League but was sent off and sat out the final game of the season at Fulham.
City face Moroccan side Wydad Casablanca in their first match of the Club World Cup in Philadelphia on 18 June
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