The 18-year-old came off the bench with 15 minutes remaining and with the score at 0-0.
It took Afena-Gyan just seven minutes to break the deadlock.
The Ghanaian forward found space inside the box and he made no mistake to notch his first goal in professional football.
Afena-Gyan immediately ran over to Mourinho to celebrate with him. It made for a wholesome moment and you can watch it below:
The 18-year-old made the points safe for his side with an absolute beauty in the fourth minute of stoppage-time.
Mourinho revealed after the game that he will buy Afena-Gyan new boots as part of a promise he made.
“I had promised to buy Felix the boots he really likes, which are really expensive, they cost €800, so he ran over and told me not to forget! Tomorrow morning, the first thing I do is buy him the boots,” Mourinho told DAZN, per football-italia.
“I am sorry for Mister De Rossi and the Primavera squad, but Felix will stay with us. The Primavera work close to us, so Felix and another four or five players have come across to the senior squad.
“What impressed me the most was his cool head in front of goal. If he is not in terms of technique fantastic, he does have a strong mentality.
“You get these kids now who think they know everything, but he is humble and you can sense he just absorbs all this information from everyone around him. That’s fantastic.”
Afena-Gyan, meanwhile, raved about his manager after the game.
“Mourinho is a great person, a great manager and a great coach. He gives you the motivation to learn every day. I am very happy he is here,” per football-italia.
“Tonight is a dream come true. It was my first goal, I was hoping for a very long time. I want to continue my performance, prove myself and do more in future.”