Diomande missed Leipzig's 4-0 win over third-tier SC Verl and was subsequently ruled out of the club's pre-season training camp in Austria, with Leipzig citing an infection.
"I know this is a big topic. Yan is genuinely ill," Demichelis told Sky before the decision on the camp was confirmed, adding that he had not yet spoken to the club doctor:
"We'll see. If he's healthy, he has to come with us. If he's ill, then he's ill."
The 19-year-old is widely understood to have mentally moved on from Leipzig and is preparing for a switch to Real Madrid, who have been trying to agree a fee with the Bundesliga club for weeks.
RB Leipzig are believed to be holding out for more than €100m, a valuation that reflects Diomande's rapid rise since arriving in Germany less than a year ago; the Ivorian has directly contributed to a goal every 89 minutes in the league this season, remarkable output for a player who only turned 19 this month and made his professional debut for Leganes as recently as last year.
Not everyone is convinced illness is the full story. Several fans questioned Leipzig's explanation on social media, pointing out that keeping Diomande out of both the friendly and the training camp is also the safest way to protect a transfer worth nine figures from an untimely injury.
Leipzig's stance towards Real Madrid has been notably frosty throughout the process, with the club yet to formally respond to a written offer from the Spanish side, even as Madrid continue to push for a swift resolution before their own pre-season plans are finalised.
Diomande was not the only notable absentee against Verl: long-serving goalkeeper Peter Gulacsi also missed out as he closes in on a move to Villarreal, while Demichelis, now in his first summer in charge following Ole Werner's departure, also had to make do without Willi Orban, Eljif Elmas, Max Finkgräfe and David Raum, all managed through individual training loads.
Leipzig won comfortably in the end, with goals from Lukas Klostermann, Ridle Baku and Ezechiel Banzuzi before a stoppage-time penalty completed the scoring.