CINCINNATI — If not for a rookie pitcher, the Chicago Cubs wouldn’t be where they are today: First place by a percentage point.

Rookie right-hander Randy Wells took a shutout into the eighth inning Monday night, and Mike Fontenot hit a three-run homer off slumping Aaron Harang, leading the Cubs to a 4-2 victory over the Cincinnati Reds.

After stumbling through the first half of the season, the Cubs have gone 20-10 since July 1. They’re 13-5 since the All-Star break, the best mark in the majors. The latest win moved them back into first place in the NL Central, a percentage point ahead of St. Louis.

‘‘We’re right there in the standings,’’ Fontenot said. ‘‘Hopefully we’ll keep winning ballgames, and all of us can get hot the last couple of months, and we’ll see what happens.’’

Wells (8-4) has been a big part of the turnaround. The 26-year-old was called up on May 8 and has been one of the league’s top newcomers. He allowed six hits, including Alex Gonzalez’s solo homer with one out in the eighth, while getting his fourth straight victory.

‘‘He’s pitched exceedingly well,’’ manager Lou Piniella said. ‘‘What a nice job, huh? Where would we be without this young man?’’

As the wins pile up, Wells tells himself not to get caught up in the success. He made it to the majors for the first time last September, when he was called up and made three relief appearances.

His motto?

‘‘Remember where you came from,’’ Wells said. ‘‘Remember where you were a few months ago. Don’t try to throw shutouts. Don’t try to win Rookie of the Year.’’

Fontenot homered in the second inning off Harang (5-13), who leads the majors in losses. Harang hasn’t won since May 25, going 0-9 in his last 13 starts. It’s the deepest slump of his career and the longest losing streak by a Reds pitcher since Danny Graves lost 10 straight between the 2003 and 2004.

‘‘We need a victory, but if anybody needs a win, it’s him,’’ manager Dusty Baker said.

Heading into the series, Piniella had one concern. Closer Kevin Gregg blew both of his save chances in Florida over the weekend and reported that his pitching arm felt tired. Piniella wanted to give him a couple of days off to recharge.

Carlos Marmol gave up a run-scoring double by Gonzalez with two outs in the ninth, leaving runners at second and third. He retired Willy Taveras on a grounder, earning his fourth save in eight chances.

While the Cubs have made a U-turn at midseason, the Reds have gone the other way. They have dropped 13 of 14 and 19 of 23, consigning them to last place in the NL Central.

The Cubs know they’re in good shape when their run total reaches three. The Cubs are 53-15 when scoring at least three runs, the best mark in the majors. They reached the dividing mark on one swing.

Fontenot homered for a 3-0 lead in the second. Fontenot had been in a 5-for-36 slump and hadn’t homered since the last time he faced Harang, on July 24. Derrek Lee doubled home another run in the eighth off Harang, who gave up six hits and struck out a season-high 10.

‘‘It just didn’t go my way,’’ Harang said. ‘‘What can you do? I go out and give us a chance to win. We can’t seem to muster anything early on.’’

NOTES: Cubs 3B Aramis Ramirez, who was hit on the left forearm Saturday and sat out the next game, was back in the starting lineup and went 0 for 3, ending his nine-game hitting streak. … Cincinnati fell to 1-7 on its 10-game homestand. … Gonzalez was out of the Reds’ starting lineup. The shortstop was in an 0-for-20 slump when he homered in the eighth. … 3B Scott Rolen had a headache and was out of the Reds lineup Monday, a day after he was hit in the head by a pitch from Jason Marquis. … Reds reliever David Weathers was ejected by plate umpire Paul Emmel for arguing balls and strikes on his way to the dugout after the eighth inning.

Reds’ boxscore

Monday’s Game

Cubs 4, Reds 2

Chicago Cubs Cincinnati

ab r h bi ab r h bi

Fukdm cf 3 1 1 0 Tavers cf 5 0 0 0

Theriot ss 3 0 0 0 ARosls 3b 4 0 1 0

D.Lee 1b 4 0 1 1 Votto 1b 2 0 0 0

ArRmr 3b 3 0 0 0 BPhllps 2b 4 0 1 0

Bradly rf 2 1 0 0 L.Nix lf 3 1 1 0

ASorin lf 4 1 1 0 Gomes rf 4 0 1 0

Fontent 2b 4 1 2 3 Hanign c 3 0 2 0

Grabow p 0 0 0 0 Janish ss 3 0 0 0

Marml p 0 0 0 0 Wethrs p 0 0 0 0

K.Hill c 4 0 1 0 Herrer p 0 0 0 0

R.Wells p 3 0 1 0 Balentn ph 1 0 0 0

JeBakr 2b 1 0 0 0 Harang p 2 0 0 0

ABlanc 2b 0 0 0 0 AlGnzlz ss 2 1 2 2

Totals 31 4 7 4 Totals 33 2 8 2

Chicago 030 000 010 — 4

Cincinnati 000 000 011 — 2

DP—Chicago 2, Cincinnati 2. LOB—Chicago 6, Cincinnati 8. 2B—Fukudome (23), D.Lee (20), R.Wells (1), Ale.Gonzalez (11). HR—Fontenot (9), Ale.Gonzalez (3). S—Theriot.

IP H R ER BB SO

Chicago Cubs

R.Wells W,8-4 7 1-3 6 1 1 2 5

Grabow H,17 2-3 0 0 0 0 0

Marmol S,4-8 1 2 1 1 1 1

Cincinnati

Harang L,5-13 7 1-3 6 4 4 2 10

Weathers 2-3 0 0 0 2 1

Herrera 1 1 0 0 0 0

HBP—by Grabow (Votto), by Harang (Bradley). WP—R.Wells. Umpires—Home, Paul Emmel; First, Gary Darling; Second, Todd Tichenor; Third, Bill Hohn. T—2:47. A—22,222 (42,319).