Professional baseball career
Minor leagues (1990–1995)
After signing his contract in Panama with the Yankees organization, Rivera—who spoke no English at the time—left home for the first time. He flew to the United States to begin pitching for the Rookie level Yankees of the Gulf Coast League, a minor league affiliate of the New York Yankees. At that point in his career, he was considered by scouts to be a "fringe prospect" at best, but he made progress with a strong 1990 season for the GCL Yankees. Pitching mostly in relief, he allowed one earned run in 52 innings pitched—a 0.17 earned run average (ERA)—and he allowed only 24 baserunners. His seven-inning no-hitter on the season's final day "put him on the map with the organization", according to manager Glenn Sherlock. The following year, he ascended to the Class A level Greensboro Hornets of the South Atlantic League, where he started 15 of the 29 games he pitched in. Despite a 4–9 win–loss record, he recorded a 2.75 ERA in 114 2⁄3 innings pitched and struck out 123 batters while walking only 36 batters. New York Yankees manager Buck Showalter took notice of Rivera's strong strikeout-to-walk ratio, calling it "impressive in any league" and stating, "This guy is going to make it.
In 1992, Rivera was promoted to the Class A-Advanced Fort Lauderdale Yankees of the Florida State League (FSL). He started 10 games in Fort Lauderdale, compiling a 5–3 win–loss record and a 2.28 ERA. He attempted to improve the movement on his slider by snapping his wrist in his pitching motion, but he inadvertently caused damage to the ulnar collateral ligament in his throwing elbow. He had elbow surgery in August 1992 to repair the damage, ending his season and briefly interrupting his minor league career. It was expected that he would require Tommy John surgery, but during the procedure, Rivera's doctors determined that he did not need ligament replacement. His rehabilitation coincided with the 1992 expansion draft to fill the rosters for the Florida Marlins and Colorado Rockies expansion teams. Rivera was left unprotected by the Yankees but was not drafted. He successfully rehabilitated his arm in the early part of 1993 and resumed pitching that year. He first joined the Rookie level Yankees to make two abbreviated starts, before returning to the Class A level Hornets to start 10 more games.
In 1994, he ascended from the Class A-Advanced level Tampa Yankees of the FSL to the Double-A level Albany-Colonie Yankees of the Eastern League, and then to the Triple-A level Columbus Clippers of the International League. For the year, Rivera finished with a strong 10–2 record in 22 starts, although he struggled with Columbus, recording a 5.81 ERA in six starts. He began the 1995 season with Columbus with the ranking of ninth-best prospect in the Yankees organization by Baseball America.At the time, he primarily threw fastballs, although his pitching repertoire included a slider and changeup as secondary pitches.
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