On the one hand, Cook had been notified before the game that he had been selected to his first All-Star Game, but on the other hand, he had taken the loss after giving up seven runs in 7 1/3 innings.
"It's kind of bittersweet," Cook said. "I wish I could have done better."
Colorado snapped its season-high five-game winning streak. But despite the loss, the Rockies (37-52) finished their homestand 5-2 and put themselves in a position to make a second-half run as they begin a seven-game road trip in Milwaukee before the All-Star break.
"One game's not going to take away from the good feeling we've created," manager Clint Hurdle said. "We're in a better position at the end of the homestand than we were when we came in. Now the next step is to go get after the Brewers."
After winning the first three games of the series, the team looked fatigued on Sunday.
"There's only so many punches to throw in certain games," Hurdle said. "Today we just got underneath them and couldn't get back on top."
Marlins pitcher Chris Volstad (1-0) earned the victory in his Major League debut after pitching two shutout innings of relief.
Even though Cook (11-6) clearly didn't have his best stuff, Hurdle kept his ace in the game until Cody Ross hit a two-run home run in the eighth inning that gave the Marlins a 7-4 lead.
"Really, I think I could have lived with the outing had I not given up the home run to Ross right there," said Cook, who labored through 110 pitches on Sunday after throwing just 79 pitches in his first-career shutout against the Padres on Tuesday.
The Rockies got on the board in the second inning when Brad Hawpe, who homered Saturday, hit his 13th homer after a nine-pitch at-bat. The Rockies grabbed a 4-3 lead in the fourth after Cook, who has done double-duty as a pinch-hitter this season, doubled to center to bring home Jeff Baker. Ryan Spilborghs then hit a broken-bat single to score Hawpe and Cook.
Spilborghs finished the series with eight hits, including three doubles and two home runs. He also had seven RBIs and scored seven runs. Spilborghs was filling in for Willy Taveras, who has been hampered by a sore left quadriceps.
"It's just looking for fastballs down the middle," Spilborghs said. "I'm just trying to be patient and get a good pitch to hit."
Cook retired the side in order in the fourth and fifth, but ran into more trouble in the sixth, when he gave up four hits in the inning and the Marlins scored two to pull ahead 5-4. Ross's two-run shot to left in the eighth put the game out of reach.
Unlike a majority of the games during this homestand, Colorado struggled to produce hits with men in scoring position on Sunday. Clint Barmes, who went 0-5, hit an inning-ending fly to center with the bases loaded in the sixth, and Yorvit Torrealba lined to short with two on and two out in the seventh.
The Rockies will have to find a way to carry the recent momentum they've built onto the plane with them when they head to Milwaukee. At this point, they can't afford to slip too far in the standings and hope to play meaningful games after the All-Star break.
"It's been an up-and-down first half for us," Matt Holliday said. "We haven't played nearly as well as we thought we would. But we're still in striking distance, which is all we can ask for."
Jeff Birnbaum is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.