The Ivorian has completed his mega-money transfer move from RB Leipzig to Madrid as part of a deal that could reach £120M (€140M) if certain add-ons are triggered as a huge profit on the €20M Leipzig paid for him 12 months ago.
The 19-year-old has completed a medical in the Spanish capital and said his goodbyes back in Germany.
However, the deal was wrapped up too late for him to link up with the squad ahead of the flight to Hungary, and Jose Mourinho opted against asking him to join them as a non-playing squad member.
Caution is key over Diomande's start to life in Madrid, and Mourinho does not want to rush his new arrival, so he will stay behind this time.
Vinicius Junior, Bernardo Silva, and Brahim Diaz - who all rejoined the squad this week - are included and could make their preseason debuts with Dean Huijsen back after an injury scare.
Eder Militao, Ferland Mendy, Raúl Asencio and Rodrygo remain out through injury and the latter could now move on following Diomande's arrival.