Most importantly, Pittsburgh is fighting for its postseason life amidst a second-half slide that has featured six straight losses and 10 in 12 games. The Pirates trail the Cardinals by 2 1/2 games for the second National League Wild Card spot after Wednesday's loss against the Reds.
Beyond that, McDonald might be pitching for his rotation spot. Manager Clint Hurdle strongly implied that was the case when he spoke before Wednesday's game.
McDonald (12-7, 3.93 ERA) carried a 2.37 ERA into the All-Star break but has posted a 6.91 mark in 11 starts since. The right-hander has walked 33 and surrendered 13 home runs in 57 1/3 innings over that period. His strikeouts have taken a tumble as well.
The Pirates also have A.J. Burnett, Kevin Correia, Jeff Locke and Wandy Rodriguez in the rotation. Jeff Karstens is back from injury, although he will pitch out of the bullpen for now, and Hurdle said he wants to give rookie Kyle McPherson a start "if the opportunity presents itself."
That puts some pressure on McDonald, who will face the Cubs for the second straight time. On Saturday at PNC Park, he gave up three runs on seven hits in 5 2/3 innings against them.
"J-Mac was out there competing, and he was finishing things off so innings didn't get away from him," Hurdle said afterward. "The one thing that complicated it for him was the leadoff man getting on in five of his six innings. He had to stay in the stretch most of the night."
That game was part of a sweep the Cubs pulled off from Friday to Sunday, allowing seven total runs along the way. Pittsburgh then went on the road and was swept in Cincinnati.
Now the Pirates come to Wrigley Field, where the Cubs have been exactly a .500 team this season (34-34). Rookie Chris Rusin will make his fourth big league start, and first at home, as Chicago kicks off a 10-game stand.
The 25-year-old lefty is 0-2 with a 5.73 ERA. The Nats knocked him out with a five-run first inning on Sept. 4, but Rusin bounced back on Sunday at Pittsburgh, limiting the Pirates to one run on five hits in five innings. He walked three and struck out four.
Pirates: Strong debut for Holt
Second baseman Brock Holt extended his hitting streak to eight games with a third-inning single on Wednesday and now is batting .364 in his first 11 big league contests.
The 24-year-old, a ninth-round pick by the Pirates in the 2009 First-Year Player Draft, spent most of the season split between Double-A Altoona and Triple-A Indianapolis. Between all three levels, he is hitting .345 with 33 doubles, seven triples and three home runs.
Thursday's off-day is the Pirates' last this season. Friday's series opener is the first of 20 straight games the club will play until it closes its schedule on Oct. 3 at PNC Park against the Braves.
Cubs: Jackson, Rizzo expected back in lineup
Brett Jackson hasn't played since Friday, when he bruised his right knee slamming into PNC Park's center-field wall to make a highlight catch of Andrew McCutchen's deep drive. Anthony Rizzo suffered a bruised right shoulder on Tuesday, when he collided with the Astros' Brett Wallace on a play at first base.
Both freak injuries appeared potentially devastating at the time, but neither player was seriously hurt. The swelling in Jackson's knee has gone down, while Rizzo is dealing only with some soreness and took swings in the batting cage without a problem.
Both players were available to pinch-hit on Wednesday and should return to a starting role on Friday.
These four games will conclude the teams' season series, which is knotted at six. The Pirates took two of three in their only other series in Chicago.
McDonald has faced the Cubs six times in his career, but only once at Wrigley Field, in 2010. This season, he has given up eight runs in 11 2/3 innings against them.