Tyson Ross went 2-11 with a 6.50 ERA for the Oakland A's last year on a team that won 94 games, which seems pretty unusual to me. After all, one reason teams win 90 games is because they determine early on to jettison those pitchers who aren't performing.
Curious about how unsual Ross' feat was, I asked on Twitter how many pitchers were at least 9 games under .500 on a team that won at least 90 games.
The loyal followers came up with the following pitchers:
Kenshin Kawakami, 2010 Braves: 1-10, 5.15 ERA on a team that went 91-71 (and made the playoffs). Kawakami wasn't good, but wasn't terrible. He won just once in 16 starts. (From @ekap1)
Randy Lerch, 1980 Phillies: 4-14, 5.16 ERA on a team that went 91-71 ... and won the World Series. The Phillies didn't really have a great staff that year, but Steve Carlton went 24-9 and pitched 304 innings. Lerch was legitimately bad, giving up 178 hits in 150 innings with 55 walks and 57 strikeouts. Among pitchers who threw at least 130 innings in 1980, only two had a higher ERA. (From @BleepingFrakker)
Dennis Martinez, 1983 Orioles: 7-16, 5.53 ERA. The Orioles ran Martinez out there for 25 starts, but they won 98 games and the World Series anyway. Martinez was in the midst of his days of heavy drinking, from which he'd recover after leaving the Orioles and become one of the best pitchers in the game in the late '80s. (From @Kiko_Sakata)
Bob Sebra, 1987 Expos: 6-15, 4.42 ERA on a team that went 91-71. Sebra kept his spot in the rotation until he went 0-4 in August, but he wasn't really that bad (remember, 1987 was the crazy lively ball year). Neal Heaton went 13-10 with a 4.52 ERA on the same team. (From @MWEinNC)
Frank Castillo, 2002 Red Sox: 6-15, 5.07 ERA. This was a fun Red Sox team. Pedro Martinez went 20-4 and Derek Lowe went 21-8, so the rest of the rotation went 37-35. They won 93 games -- but underperformed their Pythagorean projection by seven wins. First baseman Tony Clark hit .207 with three home runs in 275 at-bats, so when Theo Epstein took over after 2002 ... well, the team was already pretty good, with obvious holes to fill. (From @MWEinNC)
Mike Emeigh (@MWEinNC) says that's the list, although it's also worth pointing out David Cone, who went 4-14 for the 2000 World Series champion Yankees, but that team won just 87 games in the regular season.