Boro first team coach said "I suppose it will be seen as a strange one, to go where he's going, Bulgaria isn’t a country you associate Latin American players going to."
Ricard was Boro's top scorer for two seasons running under Bryan Robson after being signed from Deportivo Cali in a £2m deal in 1998.
Campbell meanwhile has joined ex-Boro boss Lennie Lawrence at Cardiff City in a £1m deal. He has enjoyed a highly successful loan period with the Bluebirds scoring five goals in six games.
Nearer home, Mark Crossley and Luke Wilkshire have both signed new two-year deals with Boro, while Robbie Mustoe, who is now Boro's longest serving player, is still discussing terms over a new one-year deal.
Steve McClaren has continued to reduced his massive squad throughout the season and cut the wage bill by a substantial amount. Players, with first team experience, to move on include Keith O'Neill (Coventry City), Paul Okon (Watford), Brian Deane (Leicester City), Curtis Fleming (Crystal Palace), Steve Vickers (Birmingham City), Christian Karembeu (Olympiakos).