Diaz set the Reds on their way to Wembley with a well-taken finish early into the second leg at Craven Cottage, with goalkeeper Bernd Leno making a hash of his save.
It came in another eye-catching performance from a winger who has looked revitalised in recent weeks, overcoming the trauma suffered earlier in the season to recapture his dazzling form.
Suddenly, Diaz is capable of taking on his marker again, while on Wednesday night there were flashes of the skill that made him such a favourite among fans.
But while Klopp was full of praise for his No. 7 after the win over Fulham, the manager joked that he should have had Liverpool “halfway home” earlier on.
“He got Man of the Match obviously, and I told him already ‘Man of the Match should have scored two goals and must have one assist’, so then we are halfway home already!” Klopp told Sky Sports.
“But they are all exceptional, they all work extremely hard.
“This was not a night to shine or whatever, to play a bit of fancy football; this was a night to qualify for a final.”
In his post-match press conference, Klopp was asked again about Diaz and was more glowing in his praise for the Colombian.
“Very good!” he said on Diaz’s display.
“Honestly, he’s a fantastic player, absolutely fantastic. There’s absolutely no criticism, but I wish he would have set up the second and scored the third.
“Because first half it was a fantastic football situation, where he was in a similar situation from when he scored.
“But playing-wise, what a boy! The speed, the power he can generate, the technique, the combination of all that is absolutely outstanding.
“I’m really happy, but I liked Darwin’s performance today a lot as well, to be honest.
“He was maybe not involved in these kinds of finishing situations, but to defend him is a nightmare, to be honest, he was everywhere.”