Karetsas suffered circulatory problems during Dortmund's 3-2 Champions League win over Villarreal, prompting immediate concern given the broader context of recent cardiac incidents in football.
Speaking on the sidelines of training, Book moved quickly to play down the more serious possibilities while confirming the 18-year-old would sit out the visit of Paderborn as a precaution.
"It's clear that we're putting the person's and the player's health first for now. He won't be involved at the weekend," Book said.
Asked directly whether heart problems might be behind the incident, the sporting director struck a cautiously reassuring tone.
"It doesn't look at the moment like it's heading in that direction. So far we're quite optimistic," he said, while stressing further tests were still under way.
"He's doing well under the circumstances. Further examinations are ongoing, we want to get right to the bottom of this."
Belgian media reported that dehydration may have played a role in the Greek playmaker's collapse, though Book's comments suggested the club does not believe that alone fully explains what happened.
The incident occurred during a dramatic afternoon at Signal Iduna Park, where Dortmund came from behind to beat Villarreal, a result that left Spanish head coach Iñigo Pérez to reflect on fine margins despite a performance he described as one of his side's best of the season.
Karetsas has been one of Dortmund's most exciting young performers since his move to the club, and the priority now for the German side is ensuring the teenager receives a thorough medical assessment before any decision is made on his return to action, with the club stressing there is no fixed timeline being placed on his recovery.
Dortmund's cautious handling of the situation reflects standard practice in modern football whenever a player experiences any kind of on-field medical episode, with clubs typically preferring to rule out serious underlying issues through comprehensive testing before allowing a return to full training or matches.
Book's comments suggest the club is treating the matter with appropriate seriousness while also trying to reassure supporters that the early signs are not alarming. Further updates on Karetsas's condition are expected once the additional examinations Book referenced have been completed.