While Tah arrived in a Bosman transfer from Bayer Leverkusen, Diaz was signed from Liverpool for €75m.
After a fortnight together, Tah told Kicker: "During the first sessions, we were practically always on the same team. But now I also had the pleasure of facing him. It's fun because he tests your skills and, as a result, you improve."
He had also already faced Luis Díaz as an opponent, when he scored a hat-trick against Bayer in the Champions League.
Tah stated: "We've already talked about it. He appreciated our performance in the first half, but in the second half they overwhelmed us, especially him."
And he added: "He's a top-class player and also a great person: very practical, very hard-working, but with so much quality. It's a perfect combination; that's where the best players come from. He has incredible speed and is a real threat in front of goal."