He played in Major League Baseball for the Pittsburgh Pirates, Texas Rangers, Toronto Blue Jays, Chicago White Sox, New York Yankees, Washington Nationals, Oakland Athletics, and Los Angeles Dodgers.

A graduate of Mar Vista High School in Imperial Beach, California, Loaiza was signed by the Pittsburgh Pirates as an undrafted free agent on March 21, 1991.

He made his professional debut that year with the Rookie-level Gulf Coast Pirates 1991, finishing with a 5–1 record and a 2.26 earned run average (ERA) in 11 starts.

The Pirates traded Loaiza to the Texas Rangers on July 17, 1998, in exchange for Todd Van Poppel and Warren Morris.

[8] Considered for the Cy Young Award, Loaiza finished second behind Roy Halladay, ahead of Pedro Martínez and Tim Hudson.

[16] His early season troubles reached a nadir on April 29 when he gave up six runs in one plus innings to the Kansas City Royals, who had the worst record in baseball at the time; the game was not official since it was later rained out.

[17] Loaiza's poor performance was at least partly due to injury, as he seemed to have very little velocity on his fastball early in the season (throwing it at as little as 82–83 mph)[18] and was placed on the disabled list with back and shoulder problems on May 2.

[24] He made an impressive debut for the Dodgers on September 3, working seven innings and getting the win in an 11–3 victory over the Chicago Cubs, but struggled during the rest of the season, losing his last four decisions.

[29] In April 2023, Loaiza joined El Águila de Veracruz of the Mexican League as the team's pitching coach.

A statement, released by Rivera's representatives, stated that the divorce was due to "irreconcilable differences on behalf of both parties derived from private circumstances that occurred during the lapse of their two-year marriage."

[31] On June 14, 2006, Loaiza was arrested after being pulled over by police who clocked his Ferrari at 120 mph (190 km/h) on a California freeway near San Lorenzo.

After the incident, Athletics general manager Billy Beane banned alcohol in both the home and visitor clubhouse, citing liability issues.