The pair won the Bundesliga title together at Bayer Leverkusen last year.
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Wirtz left Bayer for Liverpool this summer as Xabi departed for Real Madrid.
With the Germany attacker battling to adjust and settle in at Liverpool, Xabi has asked Real Madrid chiefs to keep tabs on Wirtz's situation.
Bayer sold Wirtz to Liverpool for £125m, but the attacker has struggled to make an impact so far this season.
Xabi is aware of Wirtz's problems and is convinced he would thrive as a Real Madrid player under him.