He spent the next several seasons in the Tigers farm system. He had a record of 6-2 in 1976 with the Advanced A levelLakeland Flying Tigers in the Florida State League and an earned run average of 2.22. He split the 1977 season, spending the first half of the year at Montgomery in the AA Southern League, where he had a record of 9-2 and an ERA of 3.38 with 64 strikeouts in 104 innings of work. At Evansville Triplets in the AAA American Association, Underwood was 3-5 with a 5.22 ERA. He spent the entire 1978 season in Evansville, finishing with a 5-5 record a 4.15 ERA.
Detroit Tigers
Underwood started the 1979 season in Evansville and had a 2-3 record and an ERA of 2.81. He was called up by the Tigers and pitched his first game on May 31 against the Toronto Blue Jays at Exhibition Stadium. Making the start for the Jays was his brother, Tom. Tom found the whole thing stupid, but the brothers brought their best. Pat did not give up a hit until the fifth inning. Tom allowed four hits and struck out four in keeping the Tigers scoreless through seven. Then, Tigers center fielderJerry Morales led off the eighth with a home run. Pat pitched into the ninth before handing the game over to the bullpen with one out. For Tom, it was a complete game loss. He also started against the Chicago White Sox on the famed Disco Demolition Night at Comiskey Park, going 7⅓ innings and giving up just 1 run in what was supposed to be game 1 of a doubleheader. With the Tigers, he had a 6-4 record and an ERA of 4.59 with 83 strikeouts in 27 appearances. He spent the entire 1981 season with the Tigers, switching from starting to relieving. He had a 3-6 record that season and five saves, with an ERA of 3.59. In 1982, he was back in the minor leagues at Evansville, where he was used exclusively as a starting pitcher, and had a record of 9-8 with an ERA of 3.98, throwing 90 strikeouts in 165 innings. With the Tigers in 1983, he pitched in four games and had an ERA of 8.71. His final major league appearance was on June 21, 1983, when he gave up five runs in two innings as part of a 10-3 loss to the Milwaukee Brewers. The Tigers traded him on June 30, 1983 to the Cincinnati Reds in exchange for third basemanWayne Krenchicki. He split the season in the minor leagues between Evansville with the Tigers and at the Indianapolis Indians of the International League for the Reds. Between the two teams he had a record of 7-4 with a 4.09 ERA.
Post-Tigers career
He was drafted by the Texas Rangers on December 5, 1983 from the Reds in the 1983 Rule 5 draft, but the Rangers released him before the start of the 1984 season. He signed with the Baltimore Orioles on April 2, 1984 as a free agent. He played in 1984 for the Rochester Red Wings, the Orioles' AAA affiliate, and had an 0-2 record and an ERA of 9.00 in three appearances in relief.
