A team's win-loss record is dependent upon the whole team—and so is the pitcher's win-loss record.
Hernandez may not have won 20 games, but that's not his fault. Blame rests squarely on the run support the guy got—or didn't get—from his team.
The Mariners offense gave Felix an average of only 2.4 runs for each of his starts. What's worse, out of his 34 starts, 15 of them were backed with two runs or fewer by the team.
As manager Dave Wakamatsu said, "You have to score runs to have a ballgame."
In many of the starts he didn't win, he allowed two or fewer runs in each game and his team still couldn't get the win.
I don't care if you're Randy Johnson, Sandy Koufax and Satchel Paige all rolled into one, you can't be a winning pitcher on a losing team.