Asked whether the negotiation had taken long to complete, Castro suggested it had been anything but.
"I'd say it was actually quite quick, because from the first moment I wanted to come here. I spoke with the sporting director, he explained what they wanted from me and what they thought I could give the team. I know what I can do and how I can help the coach and my teammates. From that moment I chose Roma."
The Argentine striker was equally direct when asked about playing under Gasperini, a coach who took Atalanta to consistent European qualification before doing the same at Roma last season.
"Of course I like it. Gasperini is a coach who has shown his worth in Italy for many years: he did it with Atalanta and now with Roma, taking them back into the Champions League. I see myself wherever he wants me. I'm here to help the team and do whatever he asks of me."
On his own game, built on 105 appearances, 22 goals and 13 assists during his time at Bologna, Castro was frank about where he still needs to improve.
"I'm a player who has to improve in everything. You can always grow, maybe by putting up better numbers. But I'm someone who wants to help the team: winning back the ball, setting up an assist or making a run that lets a teammate score. I don't just look at the statistics, but at what's good for the team."
He also pointed to a habit of scoring at the Stadio Olimpico, including for Bologna against both Roma and Lazio.
"Yes, you could say that. Hopefully we can keep it up. In the last two games I've played there I've scored, I also hit the crossbar, and against Lazio I scored a brace. Now we want to keep it going, maybe with two goals in the derby."
The 21-year-old's rise was shaped well before Italy, according to Christian Bassedas, the former Vélez Sarsfield sporting director who worked with Castro as he broke into the Argentine club's first team during a relegation battle.
"When he came into the first team and was given the starting role, at a time when Vélez was fighting to avoid relegation, he showed real character despite the huge pressure involved. It wasn't a situation just anyone could have handled.
"His young age didn't hold him back — on the contrary, he was decisive for the team's survival, and not only for the goals he scored," Bassedas said, adding that the experience "tempered his character and shaped his development as a player, preparing him for any challenge ahead."
Asked whether Castro, who counts Inter's Lautaro Martínez as an idol, was ready for a club of AS Roma's stature, Bassedas had no doubts.
"Even though he's only 21, I have no doubt he's ready for Roma, because I see him ready for any challenge. I believe in him. Whether he can get close to his idol Lautaro's level in future? He has everything needed to have a great career. I believe in him."