It's pretty myopic to make those judgments based on the last 2-3 starts, isn't it? If you look at the 2-3 starts before then, the story's almost reversed.
I don't like Pelfrey either, but I wouldn't say Santana's indisputably pitched better than him. Stylistically, they're worlds apart (Santana's a strikeout pitcher while Pelfrey induces a lot of grounders), but their results are comparable...Pelfrey on the left, Santana on the right, August splits in parentheses:
ERA - 4.17 (3.62) / 4.93 (7.11)
WHIP - 1.46 (1.39) / 1.41 (1.77)
OPS - .758 (.698) / .789 (.958)
I'll give you that Santana's last two starts have been better than Pelfrey's, but go back beyond that and you're looking at some real clunkers...like, failing to get out of the third inning clunkers. Santana's given up 5+ earned runs in 5 of his 12 starts...Pelfrey's only done that 3 times all year, and he's pitched 14 more games than Santana has (and given that Pelfrey followed Nolasco in the rotation for the first 2 months of the season, he was often pitching without many bullpen arms available).