Diaz, 29, who had a stint at Swiss side FC Basel before joining Hamburg in February last year, arrived in Vigo on Thursday for a medical and to work out the final details of the deal, Celta said on their website (www.celtavigo.net).
Diaz, who was sidelined by injury last month but Celta said he would be back in action within three weeks at the most.
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The Galician club were in the market for a central midfielder after they sold Argentina international Augusto Fernandez to Atletico Madrid this month.
Celta are fifth in La Liga at the halfway point of the campaign, with 31 points from 19 matches, and host bottom side Levante on Saturday.
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