Cashner is on the disabled list because of a strained right rotator cuff, while Wells has a strained right forearm. There is no timetable for their returns.
"All is good," said Cubs manager Mike Quade, who joked with the pair about being "payroll bandits."
The Cubs will discuss their rotation options on the plane ride to Arizona on Wednesday. Casey Coleman (1-1, 7.43 ERA), who was scheduled to start the series finale against the Rockies before the game was postponed by rain, now will go Sunday against the D-backs.
That means James Russell, who has been subbing for Cashner, is out for the time being. Russell dropped to 0-3 with an 11.17 ERA as a starter after Tuesday's loss to the Rockies. In his three starts, the left-hander has given up 12 earned runs over 9 2/3 innings.
One option in the Minor Leagues for the fifth spot could be right-hander Jay Jackson, who gave up one run on four hits over 6 2/3 innings in Iowa's 4-3 loss to Memphis on Tuesday. Jackson, who did not get a decision, went 3-for-3 with two doubles and threw 89 pitches.