Souness admits Diaz's impact since his January arrival from Porto has "surprised" him.
He told O Jogo: "The Premier League is very different, faster, more aggressive and intense. You have to know how to play like that.
"Díaz has not only played but also seems to enjoy that way of playing. I was surprised.
"He took a step forward as a player and asserted himself."
Souness also commented on Reds striker Diogo Jota.
He said: "He's very smart, he's not big but he scores several goals with his head because he understands how the game develops. For me he's ahead in the fight in the attack. He and (Mohamed) Salah.
"Jota is a tactically evolved player and who can play in the middle or on the wings."
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