The two-time All-Star walked five and yielded seven hits -- including a two-run homer by Joaquin Arias -- before leaving after the fourth inning. Only 50 of his 92 pitches were strikes, and he benefited from two runners being thrown out from the outfield.
"It was probably a little bit better than the last one when they were hitting a few balls but, you know, it's a rehab start," Mulder said. "I'm not concerned with that. I was concerned with throwing strikes. I had a handful of walks. I wasn't very happy with that, but there was a couple of them that were real questionable walks."
A shoulder injury has created problems with the left-hander's delivery, causing him to throw at three-quarters instead of over the top. After the third inning, Memphis pitching coach Dyar Miller sat on the bench next to Mulder, pulling his arm over his head and making a downward throwing motion.
"It's just a matter of me being consistent with my arm slot," Mulder said. "Part of the night it was. Part of the night it wasn't."
Miller said Mulder was hitting his arm slot about 50 percent of the time but was doing a better job of keeping his arm up.
"Each delivery is a little bit different. He's like going through spring training. I think maybe with a couple side sessions, he might be ready to go up there and pitch. We'll just have to see," Miller said.
"He's making improvements over his last start. He threw the ball better -- it was coming out of his hand better, anyway. He was missing down more," he said.
Mulder returned for the fourth and gave up a double to Adam Morrissey and a single to Arias, but Morrissey was cut down at the plate by left fielder Shawn Garrett's throw. Mulder ended his outing with his third strikeout, getting Jamie Burke swinging on a full count.
Four of Oklahoma's five runs were earned, and all of the RedHawks' hits were solid. Two other sharply hit balls were snagged by Memphis fielders -- a line drive to third by Eric Young in the first inning and a sinking liner that right fielder Brian Martin dove to catch in the second.
It was the third rehab start for Mulder since he went on the disabled list on June 21, a day after giving up nine runs in 2 1/3 innings against the White Sox, with an injury described as rotator cuff irritation and impingement in the left shoulder.
Mulder said he would fly to Chicago, where the Cardinals were to start a three-game set against the Cubs, on Friday. He said he didn't know whether a fourth rehab start would be necessary.
"I think he's pretty close," Miller said. "It's just muscle memory. I think he got in some bad habits the last year or so and it's just going to take a little while to change that muscle memory, and that's what he's going through now."
Mulder walked the first batter he faced on six pitches and got his first out Young's line drive. Adam Hyzdu lined a single to left, and Burke followed with a two-run double to the warning track in left center, the deepest part of the ballpark at 415 feet.
Young opened the third by walking on a full count, and Arias powered Mulder's 3-1 pitch into the bullpen in left center for his first home run since May 30. The RedHawks made it 5-1 when Aarom Baldiris lined an RBI single to center, but Mulder got out of the inning when Burke was caught in a rundown between third and home.
"I could care less about the results," Mulder said. "I just wanted to throw strikes. I wasn't happy with the fact that I walked those guys. A few pitches I left up over the plate that they hit hard, but it was because I had gone ball one, ball two, whatever it was, and I was behind in the count."
A healthy Mulder would be a key addition to the Cardinals, who extended their NL Central lead to 3½ games Thursday with a win against second-place Cincinnati.
The winningest pitcher in the majors from 2001-05, Mulder started the year 5-1 and had a 3.69 ERA after throwing 8 1/3 shutout innings against the New York Mets on May 17. But then the struggles started.
Over his next six starts, Mulder had an 11.39 ERA and lost four of his five decisions. He had a 6.09 ERA when he went on the DL.
In his first rehab start, he gave up two runs -- one earned -- in five innings Aug. 6 for Class A Quad Cities. His second rehab start wasn't as smooth. In four innings with Memphis, he gave up four runs on four hits.
Mulder said pain wasn't a problem for him Thursday.
"My biggest concern is getting my arm ready to go out there and be able to throw 90 to 100 pitches in a game," Mulder said. "I want to have the arm strength to be able to compete out there. My arm, as far as arm strength goes, is fine."
Mulder is a free agent at the end of the season.