Ajax midfielder Nouri's diagnosis on Friday (AEST) was worse than expected after the hospital in Innsbruck had said they were "cautiously optimistic" two days ago when a CT scan and other neurological tests did not show any abnormalities.
The 20-year-old was admitted after collapsing during Sunday's (AEST) friendly against Werder Bremen in Hippach, Austria – with the game subsequently abandoned – before being placed in an induced coma.
Former Ajax coach Bosz, who was named Borussia Dortmund boss in June, told reporters before flying out for the Bundesliga side's tour of the Far East: "I do not know what to say, I'm totally shocked.
"Abdelhak Nouri is just 20 years old and we worked with him for a long time.
"[He was] a great player, but also a wonderful person. This diagnosis hurts us all."
Nouri made his Eredivisie breakthrough with Ajax last season, making nine appearances for the first team under Bosz. But it was with Jong Ajax where he truly shone in Netherlands' second-tier Eerste Divisie, scoring 10 goals and supplying 11 assists in 26 matches for Marcel Keizer's side.
Tributes also poured in from elsewhere with former Netherlands striker Ruud van Nistelrooy tweeting: "What terrible news. My thoughts and prayers go out to Abdelhak Nouri, his family, his friends and to AFC Ajax."
Manchester United's former Ajax defender Daley Blind said: "My thoughts are with you, your family and everyone involved."
And former Barcelona striker Patrick Kluivert said: "Heartbroken to hear this news. All our prayers to Appie and his family and friends."