The Liverpool boss was speaking after his side were beaten 3-1 by Lyon as they returned to action in Dubai.
Many were pleased that the mid-season break for the World Cup initially meant that Diaz would only miss a handful of Liverpool matches.
But with reports in Colombia claiming the winger is set to undergo surgery and could be sidelined until March, Diaz looks set to miss some key Liverpool games in the coming weeks and months.
Klopp is yet to place a timescale on the 25-year-old’s recovery, but speaking to reporters on Sunday, said the injury was “a big disappointment.”
“Yeah, he was not in the best possible place,” Klopp said in his post-match press conference.
“It’s clear it’s a big disappointment for all of us, for him as well.
“It was a non-situation in training, honestly – nothing, felt something. [He] didn’t feel a lot next day but we wanted to be really cautious and said, ‘OK, come on, let’s have a look.’
“Yeah, then the news came and it was a proper smash in the face. But that’s it now.”
Darwin Nunez was used in that position prior to the break, with the Uruguayan due to reconvene with his Liverpool team-mates in Dubai on Monday.