Youmans made his major league debut in the middle of the following season, starting on July 1, 1985, against the St. Louis Cardinals.[3] For the season, Youmans went 4-3 with a team-best 2.45 ERA. One of those wins came on the second to last day of the season against Ron Darling and the New York Mets in front of 45,404 at Shea Stadium.[4]
On June 18, 1986,[5] Youmans bested Gooden the first time the two faced off against each other, holding the Mets to only one earned run in 5.1 innings, and drawing a walk in his first at-bat against Gooden.[6] However, they would face off again on August 1, and this time Gooden would emerge victorious.[7] In the hard-throwing right-hander's first full season as a member of the rotation full-time, Youmans threw five double-digit strikeout games and finishing third in the National League with 202 strikeouts. He also led the league with 118 bases on balls, however, limiting his effectiveness and contributing to a 13-12 win–loss record.
Youmans' 1987 season was plagued with injuries, as he visited the disabled list three times.[8] However, he still managed to make 23 starts for the Expos and went 4-1 with a 1.13 ERA and three shutouts in the month of July to earn National League Pitcher of the Month honors.
Drug rehab
After the 1987 season, Youmans checked into an alcohol rehabilitation center under speculation that he had been forced to do so by Commissioner Peter Ueberroth.[9] Youmans was 3-6 with a 3.21 ERA when his 1988 season was cut short on June 25 by an indefinite suspension for "failing to comply with his drug-testing program."[10] On August 10, the suspension was reduced to sixty days, and the following day, Youmans admitted during a news conference to having used cocaine.[11]
Philadelphia Phillies
On December 6, 1988, Youmans was traded to the Philadelphia Phillies with Jeff Parrett for Kevin Gross. His 1989 season was marked by injuries, as he was on the disabled list for most of May and pitched his last game on June 24, ending the season with a 1-5 record and 5.70 ERA. He had arthroscopic shoulder surgery on August 22,[12] and never pitched at the major league level again.