In his first time throwing off a mound in nearly six weeks, Braves reliever Jonny Venters had discomfort in his troublesome left elbow and cut short the planned session of 15-20 pitches Thursday.

“Just didn’t feel very good,” said the left-hander, who got a platelet-rich plasma injection from Dr. James Andrews on April 2, in hopes of avoiding surgery on the elbow he injured in the last week of spring training.

After following the doctor’s orders not to throw for four weeks to allow the cutting-edge medication to take effect, Venters resumed a flat-ground throwing program April 30 and was encouraged by how his elbow responded to those gradually increased sessions.

But when he got on the mound Thursday at AT&T Park, he felt something in the elbow and stopped after 10 or 11 pitches.

“Just a little tender,” he said. “We’ll see how it feels tomorrow, how it responds.”

Venters was asked if thought perhaps it was discomfort caused from moving up from flat-ground throwing to the additional slope of throwing on the downhill slope, along with throwing a little harder from the mound.

“You’re putting new stress on it, different stress than you are when you throw flat ground,” he said. “Might be one of those things where you’ve just got to get used to throwing off that mound again. It didn’t feel great, but see how it feels tomorrow.”

“I was putting something on it, for sure. Ball’s still coming out (of his hand) decent, just didn’t feel very good. So we’ll see.”

Venters had “Tommy John” elbow surgery after the 2005 minor league season to reconstruct a torn ulnar collateral ligament in his pitching elbow. Andrews performed that procedure.

Heyward starts rehab assignment: Two-and-a-half weeks after having an appendectomy, Jason Heyward began a minor league rehab assignment Thursday and could be back with the Braves early next week.

He’ll miss out hitting in one of his favorite ballparks: This is the Braves’ only visit of the season to San Francisco, where he has a .395 average with six homers and 12 RBIs in 10 games at AT&T Park.

Since Heyward went down, Jordan Schafer and Reed Johnson have thrived splitting right-field duties.

Schafer, who has also played some at the other two outfield positions, was 9-for-33 (.273) with a .455 on-base percentage in his past 14 games before Thursday, with a triple, a homer, 11 walks and three stolen bases. Johnson was 7-for-23 (.304) with three doubles in his past 10 games.

Beachy nearing rehab stint: After throwing his last scheduled batting-practice session Thursday, Brandon Beachy will throw a side session Sunday and begin a minor-league rehab assignment next week, Braves manager Fredi Gonzalez said.

The right-hander is in the 11th month of rehab from Tommy John elbow surgery and is on schedule for a return to the majors in mid-June, within one week of the one-year anniversary of surgery.

Beachy ranked among major league leaders with his 2.00 ERA in 13 starts before he got hurt last summer. Gonzalez said he would make about six minor league rehab starts, going on the same type of schedule that Braves starters follow in spring training.

The Braves haven’t said what the plan is for when Beachy is ready to return to the major league rotation. Rookie Julio Teheran is the current fifth starter and had a 5.08 ERA and .333 opponents’ average before his start in Friday night’s series opener at San Francisco.

The Braves’ other four starters have ERAs between 2.96 (Mike Minor) and 3.83 (Tim Hudson), and opponents’ averages of .250 or below.

Etc.: Catcher-left fielder Evan Gattis wasn't in the Braves' lineup Thursday and probably won't be Friday. Gonzalez said he wanted the rookie to get accustomed to the windy, spacious outfield for a couple of days before a likely left-field start Saturday. Brian McCann was expected to catch Thursday, Friday and Sunday, and Gerald Laird is set to catch Saturday.