Both Diaz and Darwin Nunez were missing for today's preseason friendly defeat to AC Milan in Hong Kong.
Afterwards, Slot said of Diaz's absence: "He's training really well with us, but for now we've decided not to play him yet. I'm used to speaking regularly with most of my players.
"As I said, Lucho is training very well, and we've decided he won't play for the moment. Was he excluded because of the speculation about his future? Yes, but I can't comment much more on that."
Bayern Munich have reportedly submitted a new offer - higher than their previous €67.5m - for Diaz. Negotiations between the two clubs are ongoing, and the player is expected to sign a contract until at least 2029, an agreement reached after Barcelona withdrew from the race.