Signed by the NBA
Thanks to his notable NCAA Division I college 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.
Europe
Rivas played for Taugrés in the Spanish League from 1989 until 1996, winning the Spanish King's Cup in 1995. He arrived with the Vitorian team to the FIBA European Cup (later called FIBA Saporta Cup) Finals in three consecutive years (1994, 1995, 1996) finally winning it in the 1995–96 season, against Peja Stojaković's team PAOK, and becoming the Finals MVP, with 32 points and 15 rebounds. In the 1996–97 season, he won the Spanish league with FC Barcelona and also finished runner-up of the 1996–97 FIBA EuroLeague, losing to Olympiacos from Greek Basket League. The following season went to Athens to play for AEK of Giannis Ioannidis, one of the biggest clubs in Greece, and played in another EuroLeague Final (1997–98 FIBA EuroLeague) against Ettore Messina's Kinder Bologna. In 1998, he went back to Spain, for his final year with the club Cáceres. In 1999, he played in Italy for the club Fabriano.