The 6-foot-6 Wacha (3-3) had lost his last three decisions since an April 13 win over the Cubs. The 22-year-old right-hander allowed seven hits, including a homer, but did not walk a batter. He struck out five.
Trevor Rosenthal pitched 1 2-3 innings for his 11th save. Rosenthal, who blew a save Tuesday, inherited a one-out, bases-loaded situation and gave up a sacrifice fly, but closed out the inning and retired the side in the ninth.
The Cubs have lost nine of their last 11 and are off to the worst start in 39 games (13-26) since 2002.