The Cubs need to make a roster move on Saturday when they activate Randy Wells from the disabled list to start against the Pirates. They are expected to send Casey Coleman to Triple-A Iowa.
Manager Mike Quade was hesitant to use Lopez out of the bullpen on Friday.
"I think we have to look at what he's been doing lately," Quade said after the game. "He wants to pitch and I'm sure he's versatile, but he's been starting all year."
Lopez, 35, didn't expect to be playing for the Cubs.
"I was surprised," Lopez said. "I was talking with my agent 10 days ago about [whether] teams were interested in me, and he said nobody was interested."
Lopez said he and his agent had talked earlier this season with the Orioles, Pirates, D-backs and Rockies, but not the Cubs.
Lopez was 6-1 with a 2.59 ERA in nine starts for Triple-A Gwinnett before being traded. He has a career 4.85 ERA in the Majors and has made 227 appearances over nine seasons, 199 as a starter.
"I've been pitching as a starter [this season]. I don't know what kind of role I will have here," Lopez said. "I work a lot of innings [and am] very durable. So far my season in Triple-A has been good, and I'm pretty comfortable right now. Wherever they need me, I'll be happy to help them out, regardless if it's the rotation or in the bullpen."
Lopez will likely throw a side session on Saturday in preparation for Monday's game.
The Cubs have been forced to piece together the rotation because of injuries to Wells, Andrew Cashner and Matt Garza. Lopez was signed to a Minor League contract by Atlanta in the offseason and acquired in exchange for a Minor League pitcher.
To make room for Lopez on the Cubs' 25-man roster, Justin Berg was optioned to Triple-A Iowa. Berg had a 3.75 ERA in eight relief appearances with the Cubs. He was 2-0 with a 2.25 ERA in seven relief appearances with Iowa.
To create an opening on the 40-man roster, right-handed pitcher Robert Coello was designated for assignment. Coello was 3-5 with a 7.82 ERA in nine starts with Iowa.