Harang racked up 75 wins for the Reds and is 5-5 on the season with a 3.68 ERA in 2012. He hasn't faced the Reds since 2002, when he played for Oakland.
Reds manager Dusty Baker, who was in charge for three of Harang's years in Cincinnati, said familiarity with an opposing pitcher can both help and hurt a squad.
"He knows us and we know him pretty well." Baker said. "That's more up to what my batting coach discusses with them -- if he's doing anything differently, if he's the same or if he has a pattern or whatever."
Harang hasn't recorded a win in his last four outings and hasn't lasted past the sixth in his past three outings.
He'll take the mound against Reds righty Mike Leake, who hasn't had a loss since May 11, recording three wins in his past nine starts.
The 24-year-old is coming off probably his best outing with the Reds, throwing his first career complete game, holding San Francisco to nine hits and one run.
"It was nice to be able to work quick and get through the game," Leake said. "I'm not huge on complete games, but it was nice to get one finally. It couldn't happen without the defense we had today and the early start we got in the first inning."
Reds: Stubbs' struggles continues
Center fielder Drew Stubbs' slump slipped to 0-for-24 after an 0-for-4 outing Tuesday night.
Stubbs is 3-for-30 in nine games since his return from the disabled list June 25, missing nearly three weeks with a strained left oblique.
In his first two games off the DL, Stubbs racked up three hits, including one home run.
"He said he's feeling good," Baker said. "A lot of times when you're off, you come back quick and your timing is still off. Plus, we've been facing [Matt] Cain, [Madison] Bumgarner and you're going to be off sometimes whether you're healthy or not. We knew coming into this stretch we would see some good pitching."
Baker said he didn't believe Stubbs, who is batting .218 on the season, was showing effects from the injury.
Third baseman Scott Rolen was out of the lineup for the third straight game Tuesday because of back spasms.
Rolen, who is batting .184 on the year, was on the disabled list for more than a month earlier this season.
He is listed as day to day and seemed to be moved around better Tuesday after walking gingerly the previous two days.
Dodgers: Roster swaps may be in store
With second baseman Mark Ellis ready to jump back into the starting lineup, Los Angeles may see some roster changes in the near future.
The Dodgers are considering replacing Ellis for right fielder Andre Ethier (strained left oblique) on the active roster as early as Wednesday.
Manager Don Mattingly said he hoped Ethier would be back in the lineup by the All-Star break but is now leaning toward putting him on the DL.
"We're getting closer to a decision we have to make," Mattingly said. "He swung in the cage. He's not taking batting practice on the field today, which pushes him back. Common sense tells us to make a good decision. He felt something on one swing today."
Ellis was put on the DL after surgery on his lower leg and finished his rehab assignment with Class A Rancho Cucamonga on Tuesday. He is expected to be activated Thursday for a four-game series with Arizona.
After some slight debate, the Dodgers held third baseman Juan Uribe out of Tuesday's lineup due to a right ankle injury suffered Monday.
Mattingly said Uribe's ankle hadn't improved since Monday.
"X-rays were negative and he's day to day," Mattingly said. "I thought he'd be OK, but at that point he wasn't. He woke up sore."
Mattingly turned to Luis Cruz to fill in for Uribe, who is batting .200 on the season with one homer and 12 RBIs.
If Uribe goes on the disabled list, it would be the 17th time a Dodgers player was disabled this year.
After going 0-3 with a 6.65 ERA in April, Leake went 1-2 with a 4.55 ERA in May and notched a 2-0 record with a 2.55 ERA in June.