Cook has appeared in 12 games this season and pitched 14 1/3 innings, without allowing a run. No other American League reliever has pitched that much without yielding a run.
"That's phenomenal, especially for a rookie," Melvin said Sunday. "He continues to impress. And being the eighth-inning guy -- I don't even want to talk about it."
Because of the off-day on Monday, the A's have decided to switch starting pitchers for the upcoming two-game series against Toronto. Brandon McCarthy will now start Tuesday and Jarrod Parker will start Wednesday.
Parker (1-0, 1.38 ERA), has not give up more than one run in any of his three Major League starts and has pitched at least five innings in each. Parker is just the second pitcher since 1990, along with Rich Harden of the A's in 2003, to begin his career like that.
Since 1918, no pitcher has begun his career with four consecutive outings of five innings or more and one run or fewer.
Tyson Ross, who has been hit hard in his last two starts, worked out Sunday morning under the watchful eyes of pitching coach Curt Young, bullpen coach Rick Rodriguez and roving minor league pitching instructor Gil Patterson.
Ross is scheduled to make his next start on Thursday against the Tigers.
Fuentes' two innings of one-hit relief on Saturday night earned him his second win of the season and marked his first outing of two innings or more since June 4, 2011. Fuentes has only allowed one run in 7 1/3 innings pitched on the road this season.
Gomes' game-winning 12th inning homer on Saturday night was only his second career go-ahead hit in extra innings. The other was an RBI double on June 10, 2009 with the Reds.
"I wasn't surprised," Melvin said. "He's always prepared."