Arrieta, 35, gave up five runs in 3.1 innings before exiting. His last pitch was hit for a home run by Rockies catcher Dom Nunez. The right-hander said he felt a tweak in his hamstring as he landed on his follow-through.
"It's an unfortunate intro to the group," Arrieta said after the Padres' 7-5 loss. "I feel bad for what happened. It's not the way you want to start off with your new group of guys, especially in a situation to stop the bleeding and salvage a game in the series."
Arrieta was signed on Tuesday despite compiling a 6.88 ERA with the Chicago Cubs this season. Injuries to Padres pitchers necessitated the move as lefty Matt Strahm joined teammates Yu Darvish, Drew Pomeranz, Dinelson Lamet and Chris Paddack on the injured list. Strahm has right knee inflammation. Arrieta doesn't think his injury is serious.
The loss completed a "brutal" road trip for the Padres, who went 1-5 against Arizona and Colorado. The Rockies sealed San Diego's trip with a sweep.
"It's definitely not expected, it's where we're at," Padres manager Jayce Tingler said. "It's a brutal road trip. Never got anything going.
"We have to be better at all aspects of the game. It's frustrating. We have the guys to do it."
Despite the loss, the Padres remain 1.5 games up on the Cincinnati Reds for the second wild card position. The Reds also lost a series to a below .500 team as the Cubs took two of three games from them this week. It's a theme that San Diego has had to grapple with all season: The Padres are just 36-36 against below .500 teams. Tingler was asked if he could explain the oddity considering they're 11 games over .500 overall.
"I don't know how, except we've been inconsistent," he said. "No doubt about it. ... We haven't been great on the road. We haven't been good against some of the teams with below .500 records."
Unsurprisingly, considering their injuries, the Padres are having issues on the mound. They had the second-worst ERA in the National League over the past seven days heading into Wednesday's game. After Arrieta came out, reliever Nabil Crismatt gave up two runs which became the difference in the contest.
"There is a sense of urgency," Tingler said. "The guys are battling. They're playing hard. Once we get a couple things clicking, we have a chance to get going."
The Padres hope to do that at home where they're 40-24 this season. They host the Philadelphia Phillies and Los Angeles Dodgers beginning this weekend. Both of those opponents have playoff aspirations, but that may be a good thing. The Padres play better against better competition.
"We're a team that can really get hot at any given point," outfielder Wil Myers stated. "We can run off 10 wins in a year. We've shown that. If we want to focus on the negatives, those things will continue to pile up."
Tingler was asked where the team's state of mind was after the tough trip.
"Where we're at?" he asked rhetorically. "Pissed off. Frustrated. Those can be good things."