Prior was supposed to start the second game of the season against Cincinnati on April 5, but left-handed pitcher Glendon Rusch will get the start instead.
For two weeks, the Cubs likely will go with a four-man rotation because they have three days off in that span. But if Prior is not ready (and Kerry Wood and Wade Miller are not expected to be back until May), the Cubs will look at one of four young arms to back Carlos Zambrano, Rusch, Greg Maddux and Jerome Williams.
Left-hander Rich Hill, who is struggling with his control during spring training, is the leading candidate because he has the most major league experience (10 games).
The Cubs are also eyeing right-hander Angel Guzman, who has been beset by injuries the past three seasons but owns a minor league career mark of 27-14 with a 2.61 ERA.
Left-hander Sean Marshall has been impressive in camp (0.00 ERA in 3 1/3 innings) and could get stretched out for starting duty, but he hasn't pitched beyond the Double-A level.
Right-handed Jae Kuk Ryu (0.00 ERA in three innings) would also be considered. He has only one appearance beyond the Double-A level in his minor league career.