Castillo officially signed a seven-year, $72.5 million deal with Boston on Saturday.
"I think as he said, it's always helpful to have eyes towards what the future clubhouse environment would be," said Brodie Van Wagenen, who represents Castillo. "I think Cespedes' success as well as Puig and Chapman and other players that have come as well, especially [Jose] Abreu of late, it gave him tremendous confidence that he could succeed here.
"But as far as comfort level here, I think that once the Cespedes trade happened, it electrified the opportunity as we were narrowing our choices to a decision, because then it gave a tangible comrade that he could have in this clubhouse."
As Castillo acclimates himself to the Red Sox, he will have Cespedes nearby to give him advice on making the transition to the Majors.
"I actually spoke to Cespedes a little bit about this. He made me aware that it's basically the same game we played in Cuba," said Castillo. "He told me that success here will come down to working and grinding away at a day-to-day level."