"He was better tonight," Cox said. "Just that one inning."
The worst mistake of that one inning was a crucial one -- a five-pitch walk against Lincecum, with two outs and two runners on base. Aaron Rowand followed with a two-out double, and Edgar Renteria followed that with a two-run single.
"The biggest thing was the walk to Lincecum," Medlen said. "Rowand made me pay for it."
Asked what happened against Lincecum, Medlen didn't have an explanation. "Just a case of mechanically... A letdown... I'm disappointed about the entire thing. Just disappointed. You try to attack the pitcher.... I don't know what happened."
What made the mistake more alarming was that it was the second time in as many starts that an admittedly nervous Medlen, 23, has not been able to throw strikes to the opposing pitcher in a pressure situation.
Against the Rockies, he hit Rockies starter Aaron Cook with a pitch with the bases loaded to let in the first run in a five-run fourth inning.
"He couldn't get it close to the plate against [Lincecum], and it cost us some runs," Cox said. "You don't want to get behind that many runs against him. He's not going to give you many. He's tough."
The Braves, after winning five of the last six games on their homestand, have dropped consecutive to start their seven-game trip, and the Giants have handed them their first series loss since May 5.
They'll need a win against erstwhile Braves nemesis Randy Johnson on Wednesday to avoid a sweep. Kenshin Kawakami gets the start for Atlanta.
Lincecum (4-1) had eight strikeouts and two walks and improved to 5-0 with a 2.08 ERA in five starts against the Braves.
"I'm disappointed in the loss," Medlen said. "I'm happy I settled down after the second inning, but you can't give your team a 4-0 hole with a pitcher like that on the mound."
The Braves had two runners on base in only the second and seventh innings. Lincecum struck out Medlen to end the second inning, and struck out pinch-hitter Greg Norton to end the seventh.
Cox said Lincecum, a 2008 Cy Young Award winner, is in a class with the Braves' recent pitching greats John Smoltz, Tom Glavine and Greg Maddux.
"He's in their class, that's for sure," Cox said.
Norton said: "He's got a plus-fastball he can put on both sides of the plate, a nasty changeup he used tonight to get a lot of missed swings, and a filthy curveball. He's pretty darn good."
The Braves have lost six of their last seven games at AT&T Park, where their 12-20 record is their worst since 2000 at any stadium where they've played at least 15 games.
In his debut against the Rockies on Thursday, the bottom fell out for Medlen in the fourth inning, when he issued consecutive four-pitch walks before hitting Cook with a pitch with the bases loaded.
He left without recording an out in that inning, and Todd Helton hit a grand slam off reliever James Parr two outs later to leave Medlen with five runs allowed and an ERA of 15.00. He allowed five hits and five walks in that ragged debut, and also had two wild pitches and got charged with a balk.
Early on against the Giants, it looked like Medlen was in for another rough night and even earlier exit. He gave up four runs in the second inning, when he faced eight batters and gave up four hits and the walk to Lincecum.
The damage could have been three runs instead of four in the inning, but catcher Brian McCann dropped a strong throw from center fielder Jordan Schafer as McCann tagged the second runner on the Renteria hit.
From that point forward, Medlen regrouped and showed something, pitching more like he had at Class AAA Gwinnett to get the call to the majors in the first place.
Randy Wynn flied out to end the second inning, the first of 11 consecutive outs for Medlen before he left in the sixth.