ace pitcher; top pitcher
Baseball term for the team's most relied-upon starting pitcher, often used in sports news and commentary.
As the team's ace pitcher, he started the season opener.
エース is a more general term for the best player or ace, while 主戦投手 specifically refers to the main starting pitcher in baseball.
Compound of 主戦 (main battle, leading) and 投手 (pitcher). The term is a straightforward sports coinage.