In two spring outings, Prior has struggled with his mechanics and his breaking ball. On Tuesday, he had a lengthy side session with pitching coach Larry Rothschild, throwing 50 pitches.
"He's just getting work," Cubs manager Lou Piniella said about Friday's session. "He's going to work on the back field, so that the pitching coach can stop the game if he wants and talk to him. This is not life or death, or make it or break it. It's just a progression of sorts.
"He's working on some things with Larry and he can go down there and get his work in and then we'll schedule him for an 'A' game after that," Piniella said.
If Prior isn't ready, the Cubs have right-handers Wade Miller and Angel Guzman available.Miller, 30, made his third Cactus League start Tuesday against the Arizona Diamondbacks, and gave up two runs on five hits over four innings while striking out five. Guzman will make his first start and appear in his fourth game on Friday when he faces the Cubs' intracity rivals, the White Sox.
"We have Miller, who has thrown the ball well, we have the kid Guzman," Piniella said. "We've got enough depth."
But Prior, who was the Cubs' highly touted first-round pick in 2001 and who won 18 games in 2003, is drawing the most attention.
"I understand the whole thing," Piniella said of the constant questions. "We're trying to be as fair as we possibly can. I do understand the interest, there's no question."
What's been encouraging this spring is that Prior is healthy, and not having any problems with his right shoulder, which limited him to nine starts in 2006.
"What we're trying to do is get him ready without having to put a timetable on it," Piniella said. "We don't need to put a timetable on him. We have other people. I think Mark will know when he's throwing the ball well and we'll know when he's throwing the ball well."
"We don't have to make the decision today or tomorrow," Piniella said. "We have a few weeks left. We'll see what happens with it."
Whoever is designated the fifth starter will likely stay in Arizona while the rest of the team heads to Cincinnati to open the regular season April 2. Rich Hill, the fourth starter behind Carlos Zambrano, Ted Lilly and Jason Marquis, also is expected to stay behind to make a start at the Minor League camp on April 1.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.