Arrieta (6-2) allowed one run and three hits in his first victory since June 30 at Boston. The 6-foot-4 right-hander has a 1.73 ERA in his last 11 starts, holding opponents to a .162 batting average.
"Well, you know, going in I felt a little off," he said. "In the `pen I kind of realized it. I just didn't feel like I had a lot of life on my fastball today, but I was able to come out with a good mix of off-speed pitches for strikes, below the zone for swings and I mixed in some changeups. My cutter was pretty good today."
The Cubs won three of four in a matchup of last-place teams. The Rockies have lost four of five and 11 of 15 overall.
Pedro Hernandez (0-1) allowed three runs and six hits in 5 2/3 innings in his first start for Colorado.
Arrieta retired his first 14 batters before Wilin Rosario single with two out in the fifth. Corey Dickerson singled and scored on Nolan Arenado's double in the seventh for Colorado's two other hits against the pitcher.
"Arrieta was really good," Colorado manager Walt Weiss said, "but toward the end of his rope there, we had an opportunity but could never get to him."
Arrieta, 28, said he's learned that pitching is not all about blowing hitters away. In the past, he may have stuck with his fastball on a day when it wasn't quite there instead of trying something else.
"That's something you have to take it upon yourself as a pitcher to recognize at an early stage in the game," he said. "There are times where guys won't recognize that and they'll continue to try and find their fastball and give up hits."
Chicago scored three times in the sixth despite getting one ball out of the infield the entire inning. Starlin Castro had a bases-loaded walk, Justin Ruggiano added a sacrifice fly and Luis Valbuena drove in a run with a grounder to shortstop.
Hector Rondon got three outs for his 14th save in 17 opportunities. He retired three straight after Arenado and Justin Morneau singled to start the ninth.
CUBS MAKE TRADE
The Cubs made one trade on the non-waiver deadline day, sending utilityman Emilio Bonifacio, left-handed reliever James Russell and cash to Atlanta for switch-hitting catching prospect Victor Caratini.
Caratini, 20, was selected by the Braves in the second round of the 2013 draft. He hit .279 with five homers and 42 RBIs in 87 games with Class A Rome this season.
"He's a guy we liked in the draft a lot," Cubs general manager Jed Hoyer. "We've said it all along, catching is a weakness in our system and we don't have enough of it. He's a switch-hitting catcher who can control the strike zone and has performed well. So we're excited to get him."
Bonifacio has been playing well lately, batting .400 (14 for 35) in his last eight games. He hit .279 in 69 games with the Cubs this season. Russell, 28, is 0-2 with one save and a 3.87 ERA in 44 appearances this year.
CUBS: With Chicago headed out on the road, rookie reliever Neil Ramirez (triceps soreness) will travel to the team's spring training complex in Mesa, Ariz., to continue his rehab. The hope is that the right-hander will be ready to go when he is eligible to be activated from the 15-day disabled list on Aug. 10.
ROCKIES: Outfielder Michael Cuddyer (fractured shoulder) is scheduled to take part in light batting practice drills on Friday. If he progresses as expected in the next seven to 10 days, a minor league rehab stint will be the next step in his recovery.
The Cubs begin a six-game road trip Friday night in Los Angeles against the Dodgers. Rookie right-hander Kyle Hendricks (1-1, 2.33 ERA) faces Dodgers right-hander Dan Haren (8-8, 4.49 ERA) in the series opener.
The Rockies open a three-game series Friday at Detroit, with left-hander Franklin Morales (5-5, 5.18 ERA) taking on Tigers right-hander Justin Verlander (9-9, 4.59 ERA).
Copyright by STATS LLC and The Associated Press
CHICAGO, IL - JULY 31: Starting pitcher Pedro Hernandez #43 of the Colorado Rockies delivers a pitch during the...