Rookie Javier Assad convinced Cubs manager David Ross to pitch the eighth inning Saturday, based on his perfect seventh following a dominant performance in the World Baseball Classic for his native Mexico last month.

Even with more experienced relievers rested and available, Ross stuck with Assad until it was too late, as evidenced by the first three batters that reached base and scored to transform a one-run lead into a 3-1 loss to the rival Brewers before 31,363 chilled fans at Wrigley Field.

“I had no hesitation to send him out there,” said Ross, concurring that Assad was scheduled to face the bottom of the Brewers’ order after breezing through the middle of the lineup.

Before Saturday’s game, Ross said the roles of his relievers were “undefined,” with the likely exception of Brad Boxberger and Michael Fulmer, whose well-located breaking pitch to William Contreras still landed in front of the feet of defensive replacement Miles Mastrobuoni in right field for a tie-breaking two-run single with two out.

The Cubs’ bullpen has the flexibility of long relievers like Assad and Adbert Alzolay, who is extremely effective against right-handed batters, as well as Mark Leiter Jr. and Boxberger against left-handers.

Assad, 25, who has been a starter throughout most of his career, posted a 1.42 ERA with 12 strikeouts and three walks in 12 2/3 innings covering spring training and the WBC. His velocity also climbed from 92 mph last fall to 97 in a WBC game against the United States.

And he pitched well enough until the eighth, when he allowed a single to Brian Anderson and walked pinch-hitter Garrett Mitchell on four pitches even though Mitchell attempted to execute a sacrifice bunt early in the count.

“You hate to put it on one moment,” Ross said. “You got to take the outs when we can get them.”

A single by Jesse Winker tied the game and wiped out Justin Steele’s bid for a well-deserved victory. Steele limited the Brewers to three hits and struck out eight in sixth innings.

Fulmer was summoned with one out in a tie game and walked Willy Adames to load the bases. Nevertheless, Fulmer nearly bailed the Cubs out of further damage as his breaking pitch to Contreras appeared out of the zone.

Contreras, however, hit a flare to shallow right. Mastrobuoni, who replaced Trey Mancini to start the inning, appeared to get a late break that proved costly.

Mastrobuoni, 27, has split most of his time in the minors in the Rays’ organization at second base and the outfield but was making only his second appearance in right at Wrigley. He made no excuses for the winds that gust at 23 mph at first pitch and persisted.

“For some reason, I shut it down in my head,” Mastrobuoni said. “It was telling me to shut it down. But probably looking back on it, I definitely wanted to dive and leave all the uncertainty out there.

“I think maybe a little late on the jump, but thought I had a good track on it but it was pushing away from me.”