As a youngster, Rivas played at The San Juan Y.M.C.A, for Millin Romero. He progressed through Carolina's minor basketball tournaments, becoming one of the best centers in Puerto Rico, while he played for Levittown's Pedro Albizu Campos High School's team. Flor Melendez took note of his progress, and signed him to play for the Carolina Giants, of Puerto Rico's top-tier level league, the BSN. That year he was selected BSN Rookie of the Year.
The changes on the Carolina Giants were evident, when Rivas joined the team: From being one of the worst teams in the league, during the 1983 tournament, they got better every year. In 1987, the team almost reached the BSN playoffs, and, in 1988, the team finally reached the BSN playoffs, for the first time, having the best record in the regular season (1989 Regular Season: PPG.24.4, Reb.17.4 in 30 games).
College career
Rivas attended Temple University, where he played NCAA Division Icollege basketball with the Temple Owls, from 1984 to 1988, going on four occasions to the NCAA post season tournament. He was coached by Hall of Famehead coachJohn Chaney. Temple was ranked 1st in the nation in his senior year, with a record of 34 wins and 2 losses. Playing with the Owls helped him gain experience, and improve his game in the NCAA. He became well-known in the United States, as a center who could score points, and rebound in double figures, and was a respected player among his peers.
Professional career
Signed by the NBA
Thanks to his notable NCAA Division Icollege basketball career, the Boston Celtics announced that they would sign Rivas for a full season, towards the end of the 1988 Summer Olympic Games, in Seoul. Having coincided with José Ortiz's signing by the Utah Jazz two weeks prior, Rivas' signing by the Celtics was a cause of great celebration for Puerto Ricans, many of whom felt their efforts in basketball were finally being recognized by the NBA.
With the Celtics, Rivas had the opportunity to share playing time alongside Larry Bird, Kevin McHale, Dennis Johnson, Robert Parish, Reggie Lewis, and Brian Shaw, among others. The Celtics reached the playoffs, by beating the Washington Bullets by two games, for the eighth and final playoff spot in the NBA east that year, but were swept in three games, by the eventual champion Detroit Pistons, in the playoffs' first round.
Because of his height, Rivas had to change playing positions when he arrived to the Celtics, going from playing center in Puerto Rico, to power forward in Boston.
After that season with the Celtics, he came back to Puerto Rico, and received his first Puerto Rican League MVP honor, while playing for the Carolina Giants. That summer, he was signed by a basketball club in Spain, called Taugrés, which became his home for the next 7 years.