ST. LOUIS _ Javier Baez's left knee improved to the point where he returned to the starting lineup Friday night as the Cubs opened a three-game series against the Cardinals at Busch Stadium.
Baez hit his 20th home run in the top of the sixth inning, but left-hander Mike Montgomery was tagged for 12 hits in five innings of a 5-2 loss.
Montgomery allowed four hits during the Cardinals' three-run fourth. Yadier Molina had three hits, including an RBI double with two outs in the fourth, and Paul DeJong followed with a two-run single.
Montgomery allowed at least one hit in each of his five innings.
Anthony Rizzo cranked his 14th homer of the season, a deep drive down the right-field line off Luke Weaver, to give the Cubs a 1-0 lead in the third.
Baez started at shortstop in place of Addison Russell. Catcher Willson Contreras also received a rest.
The Cubs faced a Cardinals team that retooled its pitching staff. Prized prospect Dakota Hudson, Daniel Poncedeleon and left-hander Tyler Webb were promoted from Triple-A Memphis. Luke Weaver, who lost to the Cubs on Sunday, was promoted from Class-A Peoria and pitched six innings of two-run ball.
Weaver took the roster spot of left-hander Brett Cecil, who was placed on the 10-day disabled list because of right foot inflammation.
Reliever Sam Tuivailala was traded to the Mariners for a minor-league pitcher, and relievers Greg Holland and Tyler Lyons were designated for assignment.
