McDonald, an 11th-round pick of the Los Angeles Dodgers in 2002, had a previous career-best of five wins across 45 appearances with the Dodgers in 2009.
He won four games in 11 starts with the Pirates last season and got to win No. 9 of 2011 in a 3-1 defeat of Houston on Sept. 5.
Two losses in two starts have followed, in which McDonald has allowed 12 hits and nine runs in 7 2/3 innings against Florida and the Dodgers.
He's 3-1 lifetime against the Reds with a 3.29 earned run average in 27 1/3 innings.
For Cincinnati, lefty Dontrelle Willis takes the mound for the 13th time in search of his first victory in 2011.
The 29-year-old Oakland native was a 2-0 loser to the Pirates in a July 18 start at Pittsburgh and got a no-decision against them on Aug. 20 after allowing three runs in six innings.
He was ripped for nine hits and eight runs in a 12-8 loss to the Chicago Cubs in his last start Sept. 12.
Willis, who is 2-6 lifetime against Pittsburgh in 11 starts, hasn't won since beating Colorado, 4-3, on June 5, 2010 while with the Arizona Diamondbacks.
On Saturday, Jason Jaramillo went 2-for-4 with a double and an RBI, as the Pirates downed the Reds, 4-3, in the middle game of their three-game series.
Brad Lincoln (2-3) allowed two runs on six hits over five-plus innings to earn the win. He walked four and struck out four.
Jason Grilli stranded a runner in scoring position in the ninth to record his first save of the season and the third of his career.
Josh Harrison went 4-for-5 and was a home run short of the cycle for the Pirates, who have won three of four.
"We have been swinging the bats better," said Pittsburgh manager Clint Hurdle. "We've gone through some very dry periods and came out swinging better."
Travis Wood (6-6) lasted just 2 2/3 innings, surrendering four runs on seven hits to suffer the loss.
"In the last start of the season, you always want to finish strong," said Wood. "But it didn't happen that way."
Joey Votto doubled twice and drove in a pair for the Reds, losers of six of eight.
Pittsburgh is 10-4 versus the Reds this season.