Boston Red Sox pitcher Roger Clemens became the highest-paid player in baseball history yesterday when he agreed to a four-year contract extension worth $21,521,000.

The annual average value of the deal, which has an option for 1996, is more than $5,380,250. That surpasses the $4.7 million Oakland Athletics outfielder Jose Canseco averages in a contract signed last June.

Clemens, 28, is scheduled to make $2.5 million in 1991, the final season of a three-year, $7.5 million contract.

With the extension, he will make $4.4 million in 1992, $4.5 million in 1993, $5 million in 1994 and $5.5 million in 1995. The Red Sox have an option for 1996 at $5.5 million with a $1.5 million buyout. Clemens also gets a $621,000 bonus for agreeing to the extension.

Meanwhile, the first two arbitration cases were decided, with San Diego all-star catcher Benito Santiago losing and Montreal reserve outfielder Mike Aldrete winning.

Santiago came up short when arbiter Rolf Valtin opted for the Padres' offer of $1.65 million over the player's request for $2.5 million. Aldrete's request for $510,000 was chosen by arbiter John Caraway over the Expos' offer of $350,000.

Santiago, 25, won last year in arbitration at $1.25 million. If he had been successful this time, he would have broken the arbitration record of $1,975,000 set by Don Mattingly of the New York Yankees in 1987.

Santiago hit .270 last season with 11 home runs and 53 RBI, missing two months with a broken left arm after he being hit by a pitch.

Four players avoided arbitration by agreeing to one-year deals. Outfielder Randy Kutcher and the Red Sox agreed at a salary of $250,000, a raise of $65,000. Kansas City outfielder Jim Eisenreich and the Royals agreed at $950,000, doubling his salary. Left-hander Tom Glavine will get $697,500 from Atlanta, more than doubling his pay. The Braves also signed right-handed reliever Jeff Parrett for $855,000.

Clemens, the 1986 American League MVP and two-time Cy Young Award winner, will reach an annual salary of $5 million a year before Canseco and Los Angeles Dodgers outfielder Darryl Strawberry. Canseco is scheduled to make $5.1 million in 1995, the final season of his $23.5 million, five-year deal. Strawberry will make $5 million in 1995, the last year of a four-year, $20.25 million contract.

"We believe that Roger Clemens is the best player in baseball and deserves to be paid the highest salary in baseball," said Randy Hendricks, one of Clemens's agents.

Clemens's contract marks another step in the vast escalation of salaries over the past 16 months. Minnesota outfielder Kirby Puckett became the first $3 million-a-year player on Nov. 22, 1989, and Clemens became the 34th.

Nolan Ryan got the first $1 million-a-year contract in November 1979. George Foster became the first $2 million player in February 1982.

Clemens has a .695 winning percentage (116-51). Since he joined the Red Sox, the team has a .501 winning percentage in games he hasn't pitched.

He was 21-6 last season with a major league-leading 1.93 ERA. He is Boston's career strikeout leader with 1,424 and has 41 games with 10 or more strikeouts.

In other moves:

Toronto second baseman Roberto Alomar, traded by San Diego in the Fred McGriff-Joe Carter deal, was awarded $1.25 million by arbitrator William Rentfro rather than the Blue Jays' offer of $825,000.

Braves outfielder Ron Gant increased his salary seven-fold, agreeing on a one-year contract for $1,195,000. Gant, who earned $150,000 last season, hit .303 with 32 home runs and 84 RBI. He can earn an additional $55,000 in bonuses.

Free agent third baseman Jim Presley agreed to a one-year contract with the San Diego Padres. Presley hit .242 with 19 homers and 72 RBI with Atlanta last season.