The Argentina teammates are close and De Paul has revealed his national team skipper was the driving force behind his move from Atletico Madrid.
He said, "Leo and I always dreamed of playing together at a club, not just with the Albiceleste. Every call-up was special, but too brief. Now that dream has come true."
Having arrived on loan until the end of the year, with the option of a four-year contract as Designated Player, De Paul revealed Messi's key role in the deal.
"He called me, he took care of every detail. The day after I arrived, he was ready to welcome me."
Messi confirmed: "We know each other well, Rodrigo gives everything. He's a valuable addition, even in this difficult climate."
De Paul made his debut in the Leagues Cup victory over Atlas and already has a clear goal: "Here we play to win. This club will grow, but we need trophies."