The Rusney Castillo negotiations continue to progress rapidly, and the Red Sox are one of the three clubs most actively involved for the Cuban free-agent outfielder, according to major-league sources.

Castillo, since his showcase in Miami on July 26, has held private workouts for eight clubs, sources said: the Red Sox, Phillies, Yankees, Cubs, Tigers, Giants, Mariners and Reds.

His workouts with the Mariners and Reds took place in the past week but only because they were rescheduled for logistical reasons, sources said. The timing of those sessions is not an indication either club is a finalist.