A deal for the Costa Rica international was clinched on Friday - the transfer deadline for Serie A clubs.
Vasco Morelli told TMW: "I've known Marco (Giannitti, the GM of Frosinone) for more than 20 years. We were working for different profiles, Frosinone was looking for an international and other solutions were followed, then we got the idea of Joel Campbell.
"Giannitti had followed him for several years, I knew that Umberto Riva knew Joaquim Batica, his agent. I called him, we were good at being fast. Even at night, as I said before: Giannitti called me at 2am, we got in touch with Umberto and the agent, on the morning of the 14th they came to Rome.
"The club then asked me to start making the first direct approaches with Arsenal, until the decisive call between (Frosinone president) Stirpe and the Gunners management. At that point the negotiations ended, with a lot of compliments from the English club for the way in which it was conducted."