Justin Kluivert will wear the No. 34 shirt at Roma in tribute to his friend Abdelhak Nouri, who suffered brain damage after having a cardiac arrest while playing for Ajax last year.
Kluivert, who joined Roma from Ajax in a £17.25 million move, posted a video on Instagram in which he said he was adopting the number in honour of his "friend-brother."
Nouri, 21, was airlifted to a hospital after suffering the cardiac arrest during a friendly against Bundesliga side Werder Bremen last July but suffered "serious and permanent" brain damage.
"Why 34? Appie's number. My prayers and shirt number for you, my friend-brother who taught me so much," Kluivert said.
Last month, Ajax general manager Edwin Van der Sar said the club had not provided adequate on-pitch treatment following the incident.
"We recognise our responsibility and liability for the consequences of this," he said. "For a long time we were convinced that Abdelhak had received the best possible care on the field."
Investigations showed that a defibrillator should have been used sooner in the treatment, and Van der Sar added: "Had this happened, it's possible that Abdelhak would have come out in a better condition."