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Walker Buehler got the best welcome-back-to-L.A. surprise at the airport Thursday ... he learned from our camera guy his Dodgers squad signed A.J. Pollock!!!

"We signed him today?! We signed him while I was on the plane?!"

The 24-year-old pitching phenom took the breaking news with a smile ... and said he was stoked to have yet another All-Star on the team.

"That's cool, man. He's a good player. That's cool, man. I'm excited!"

As for if Pollock is the missing piece the Dodgers need to finally get them a World Series ring ... ya gotta hear what Buehler has to say about that.

BTW ... Walker's been away from L.A. for the past couple months -- and says he's done nothing but relax with his girlfriend (who he introduced us to!) since the World Series.