The Romania international is expected to move on to bigger things at the end of the season.
Estadio Deportivo says AS Roma, Tottenham, Bayer Leverkusen and Villarreal are all keen on the 26 year-old.
Barcelona were also interested, though have pulled back given the recent form of Eric Garcia at wing-back.
Ratiu's contract at Rayo carries a €25m buyout clause.
Related:
Tottenham Hotspur
Roma
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good luck brother