On September 14, 1991, Rivera challenged Dingaan Thobela for the WBO world Lightweight title, losing a twelve round decision to Thobela in Johannesburg, South Africa.
Battling to make the weight, he lost the title in his first defense to Orzubek Nazarov, and lost a rematch to Nazarov in 1994.
Mauricio has fought many undefeated boxers, including Shane Mosley, Dingaan Thobela, Todd Foster, Dennis Holbæk Pedersen, Billy Schwer and Rene Arredondo.
He won the WBA lightweight title in 1993 in South Africa against Dingaan Thobela and defended it on the road against contenders Thobela, Joey Gamache and Leavander Johnson and two lesser fighters in his adopted home country.