Prior was scratched before the game with a strained muscle in his left side, suffered while he was taking batting practice Saturday. The team said it was too early to decide whether the injury would put the right-hander on the disabled list for the seventh time in his five-year career.
It was tough news for the Cubs, who learned Saturday that Kerry Wood could miss the rest of the season with a partial tear in his right rotator cuff.
Rusch (3-7) allowed two runs and four hits in his first start since June 16. He struck out five and walked two. With the All-Star break following Sunday's game, Chicago manager Dusty Baker was able to use six relievers over the final four innings.
Four of the six walks issued by Brewers starter Doug Davis (5-6) scored. The left-hander pitched 5 1-3 innings, giving up seven runs and seven hits with four strikeouts.
Murton added an RBI single in the seventh and Angel Pagan's homer gave the Cubs a 9-2 lead in the eighth.
Rickie Weeks hit a solo homer and Carlos Lee had a two-run drive for the Brewers. Lee's 26th homer in the eighth cut the Cubs lead to 9-4.
The Cubs trailed 2-1 when Ramirez led off the fourth with a homer. Michael Barrett then walked, Jones doubled and Murton's sacrifice fly gave the Cubs a 3-2 lead.
Ronny Cedeno followed with a perfect suicide squeeze bunt, scoring Jones to make it 4-2.
Notes: Prior was winless in four starts since returning from the DL with a strained muscle in his right shoulder. He only has made 101 starts in five seasons. ... Brewers LHP Chris Capuano was named to the NL All-Star Team before Sunday's game, replacing Tom Glavine of the New York Mets. ... Brewers LHP Jorge De La Rosa made his third rehab start Saturday night, pitching five innings for Double-A Huntsville, allowing one unearned run and three hits without a walk. He struck out one and received the victory in the win over Birmingham. ... The three-game winning streak for the Cubs matches their season high, accomplished three other times. ... The four-game series drew 162,294 fans, with the final three games listed as sellouts. ... Pierre went 2-for-5 with three RBIs.