The Cubs have an off-day on Thursday and will skip Lopez, who would've gone Sunday. Instead, Carlos Zambrano (6-5, 4.78 ERA), Wells (1-3, 6.71 ERA) and Matt Garza (4-7, 3.80 ERA) will go in the weekend series against the Astros, which opens Friday.
Wells has not won a start since April 4, his first outing of the season. He went on the disabled list the next day because of a strained right forearm and hasn't found his groove since returning May 28.
"We'll take a close look at Randy and try to get him straightened out," Cubs manager Mike Quade said.
"He needs to make progress, and I'd like to see him pitch the way I've seen him in the past," Quade said of the right-hander, who had a solid spring and won the fourth spot in the rotation. "I'm hoping for as good an outing as I've seen from him in quite some time."
The Cubs also have Monday off before going on the road, and one option would've been to skip Wells and keep Lopez in the mix.
"Lopez has pitched really well," Quade said of the right-hander, who has a 1.83 ERA in his last three starts. "It's a commitment from me to Wells right now to say, 'Look, you've been a big part of this thing and keep working, but progress is important,' and so we'll do this for this Houston series."