Still, the postgame mood in the Pirates' clubhouse was anything but somber.
"This is nothing new, the way our season has played out," Pirates manager Clint Hurdle said. "We'll continue to battle, to go play. When we get the result we want, we put that one aside and try to go get another one. When we don't get the result we want, we wash that one and get ready for the next game.
"We have a day off in Cincinnati and we'll be ready to hit the gas pedal again on Friday," he said.
With three games remaining in the regular season for each team, the Pirates trail division-leading St. Louis by three games.
The Pirates, who already have clinched a playoff spot, hold a one-game lead over Cincinnati for the first wild-card slot. The Reds, who also are assured a postseason slot, host Pittsburgh for three games to finish up.
McDonald had three hits, including his homer in the sixth inning that put the Cubs ahead 4-1.
"That's one of the things that all of us can learn from right now, is we're trying to get where these teams are at that we're playing down the stretch," McDonald said. "I think every one of us in here wants to finish the season strong."
Pirates starter Francisco Liriano (16-8) certainly wanted a stronger finish. He was over 100 pitches when McDonald came to the plate in the sixth and already had surrendered a pair of doubles to him.
Hurdle thought about making a change, but never really gave it much serious consideration.
"He wasn't able to get the ball where he wanted on McDonald," Hurdle said. "There's never a time I don't like Frank on the mound. You always hate to have to go get a guy after the runs are given up. That's the walk of shame. That's on me."
The Pirates wasted other scoring chances. They had a runner on second with two outs in the fifth. Morneau hit a leadoff double in the sixth and advanced to third with one out, but was stranded when Pedro Alvarez grounded out and Tony Sanchez struck out.
"If we make three good outs, it's a 4-4 game going into the ninth," Hurdle said. "Just by getting outs. Runners on third base with less than two outs is one of the challenges we've had all season. We have to find a way to cash in in those situations. It's tough to keep pitching without a safety net."
While the Pirates slipped back in the division race, they do have the safety net of knowing they'll be in a playoff game next week regardless of what happens in Cincinnati.
"We just have to go there and play," Liriano said. "We're not going to try to do too much or change anything. We just have to go out there and play, game by game."
Dale Sveum doesn't know if he'll be coming back for a third season as manager of the Cubs. Sveum is under contract to return for 2014, but team president Theo Epstein has said all jobs will be evaluated after another losing season. "I'm not going to sit here and lie that you're not wondering what's going to happen in four to five days from now. That's just human nature," Sveum said Wednesday before the game. Sveum expects to hear about his future shortly after the season ends Sunday. "That's part of the gig," he said, "is knowing the day after the season." ... Marte stole three bases, the most by a Pirates player since Andrew McCutchen on June 15, 2010. ... The Cubs are off Thursday before opening a three-game series Friday in St. Louis with LHP Travis Wood (9-11) facing RHP Lance Lynn (14-10). ... Pirates RHP A.J. Burnett (9-11) takes on Cincinnati's Homer Bailey (11-11) on Friday.