The 20-year old midfielder has signed a two-year deal with the New Zealand outfit, joining from Premier League club Sunderland, where he has acted as reserves captain.
Greenacre said: "We were chatting one day and I asked if there was anyone available who was worth looking at.
"He told me about Alex and said he was surprised they were going to let him go.
"Robbie said a couple of Championship clubs had made enquiries and that if we were keen to get in touch with Alex."
Alex, who hails from Tenerife, will link up with the Phoenix for pre-season training on June 2.
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