Cashner has a mild strain in the back of his rotator cuff after leaving Tuesday's start against the Arizona Diamondbacks with stiffness in his right shoulder. Wells has a right forearm strain.
MRIs revealed no structural damage to either pitcher. Wells and Cashner won't do anything for the next two weeks and they likely will be placed on the disabled list by Friday. Cubs manager Mike Quade said he is not counting on either pitcher for three to four weeks.
Casey Coleman will be recalled from Triple-A Iowa and is expected to start Sunday against the Milwaukee Brewers. Quade said left-handed reliever James Russell might be stretched out to take the fifth spot in the rotation and start on Tuesday in Houston.
"The way I'm looking at it now is over the next three weeks we need a line of three Tuesdays, where we need pitching," Quade said. "Whether that's [Russell] coming out of the chute or whether someone takes the bull by the horn in the minor leagues and establishes themselves, we need somebody three times in three weeks."
General manager Jim Hendry said the Cubs will likely wait until Friday to recall anyone from the minor leagues.
"We have an off day [Thursday]," Hendry said. "We will kind of re-group and decide whether Casey will go Sunday or Tuesday. We may do that [Wednesday] or [Thursday] and then act accordingly."
In his first major league start, the 24-year-old Cashner threw 72 pitches in 5 1/3 innings against the Diamondbacks. Cashner yielded one earned run while striking out two and walking one.
"The last two pitches I threw to [Willie] Bloomquist [in the sixth inning] felt a lot more discomfort," Cashner said on Wednesday. "[Trainer] Mark [O'Neal] came out, and I was honest with him. We went and got it checked out. I have a little rotator cuff strain and we'll go from there."
Cashner, who was a reliever last season with the Cubs as a rookie, beat out veteran Carlos Silva for the fifth spot in the rotation this spring. Silva was not interested in going to Triple A to serve as a backup in case of injuries on the big league staff, so the Cubs bought out his contract.
Wells beat the Diamondbacks in his first start of the season on Monday, allowing one earned run in six innings. He said the discomfort in his forearm first surfaced during his final start of spring training.
"I was feeling really good in camp. My body was in great shape," Wells said. "During my last tuneup [in spring training] I felt a little soreness [the forearm]. I came in the next day and told the trainers about it. I got some treatment, did what I had to do. Then I threw my side and felt great.
"I also threw my side before my start and felt great. Warming up for the start I felt great. But after the game and toward the middle I felt a little uncomfortable. Afterwards, I came in and the plan was to see how I felt the next day. I came in [Tuesday] and felt a little sore. I went and got it checked out and it turns out to be a little sprain."
Wells said he is not going to rush a return to the rotation.
"I'm going to make sure I'm completely healthy and start a throwing program to make sure I'm 120 percent when I go back," he said.
Hendry said the strain in Wells' forearm is nowhere near his elbow.
"Nothing even close to the elbow. Nothing at all," Hendry said. "... but when you hear forearm [you think elbow]. A flexor strain or muscle strain, you think it might become an elbow. But there is nothing wrong with the ligament and there is nothing wrong with the ulnar nerve."
Bruce Levine contributed to this report.