Padilla told Mediotiempo.com: "Negotiations are quite advanced. I think they are heading the correct direction as it is quite interesting. I knew it was possible to happen and it is taking better and better shape."
Padilla added that Betis have priority in the negotiations, as the first side to have expressed an interest.
"We want to be fair with Betis as the first option," he added. "Next week it will be defined between Betis or Roma."
Castillo's contract with Shakhtar is ongoing and any move would be on loan.
"We obviously have a lot to analyse and to take care of," Padilla said. "Even though Roma will play the Champions League and they mean a lot, we also have to consider what they are going to offer, whether Nery will be a key player for them or not.
"The decision depends on ourselves. We must be careful. It is not a matter of money."