Drafted by both the Indians and the NBA St. Louis Hawks, Siebert was an outfielder his first two pro seasons before taking the mound in 1960. A late bloomer, he developed control and a devastating curveball, and blossomed with 16-8 records in both 1965 and 1966. His .667 winning percentage in ’66 led the AL. He no-hit the Senators that June 10, and threw two scoreless innings in the All-Star Game.
After two injury-marred seasons, Siebert went to the Red Sox in a controversial but beneficial 1969 trade for Ken Harrelson. He led Boston’s starters in winning percentage in 1970 and 1971, going 15-8 and 16-10. He dropped to 12-12 in 1972 after he injured his ankle climbing rocks at the seashore. Pitching for St. Louis on September 11, 1974, he beat the Mets in the second-longest game, by innings (25), in NL history. Siebert hit six of his 12 career home runs in 1971, including two in one game.