Wells, sidelined with a strained right forearm, will pitch in an extended spring game on Thursday in Mesa. Cashner, out with a strained right rotator cuff, will have another live batting practice session Thursday.
There is no timetable for their return, but both are expected to make at least one Minor League rehab start, if not more, before they are activated.
Casey Coleman (1-2, 6.29 ERA) is filling one of the vacancies, and the Cubs need a fifth starter on Saturday against the Giants. Cubs manager Mike Quade said he will announce on Wednesday whether they will go with James Russell, who is 0-4 with a 10.05 ERA in four starts.