Prior, limited to nine starts last season because of problems with his right shoulder, threw 40 pitches over two innings Saturday against the Kansas City Royals in his second Cactus League game and didn't look sharp. He didn't throw any breaking pitches for strikes, and had difficulty locating his fastball.
"His confidence is playing with him a little bit," Piniella said Sunday. "I told him, 'Look, you have to relax.' I think this is the right approach. If I didn't think it was the right approach, I wouldn't send him down there. He can go down there and relax and just pitch. He doesn't have to answer to the media. Leave him alone, let him pitch."
Prior wasn't bothered by the assignment, scheduled for Thursday at Fitch Park, or the meeting with Piniella.
"He has meetings with everybody. That's what managers do," Prior said.
Cubs pitching coach Larry Rothschild said it's just a matter of time before Prior gets his mechanics aligned correctly.
"The thing about it right now is that he's done everything," Rothschild said. "He's been able to do the sides, and he's had no problems, and if we keep going like that, he's going to get better and better. You just don't want any setbacks. You don't like to see the performances what they are now, but I think if we can get through some of this and keep him healthy, he'll get better and you won't worry about what happened the first two times out."
In two outings this spring, Prior has given up seven runs on eight hits and five walks over 3 1/3 innings, and has not struck out a batter.
"No one gets to this level unless you expect a level of performance out of yourself," Prior said. "Everybody does that. Whether it's Spring Training or not, you don't like to give up runs. Granted, it doesn't go on your record or the back of your baseball card, but you still expect a level of performance."
The problem isn't his arm, it's his mechanics.
"Things haven't felt as good as I would've hoped to feel," he said. "You just keep working. [The media] are looking for an answer or something to latch onto. I don't have something to give you guys. It's more a feeling, kind of what you know and what you feel. There's nothing I can say to help you guys simplify it or whatever. There's nothing I can put into words, I guess. It's something you know and you feel. You know what you've got to do and you know where you've got to be."
The good news is that it's March 11, and Piniella reminded Prior that there's plenty of time to make a decision.
"Like I told him, I said, 'Look, you have to get your breaking ball over, you have to incorporate your changeup,'" Piniella said. "I asked him how he felt about last year, and he said he didn't feel good about last year. I said, 'Well, you pitched 44 innings at the big-league level and gave up 46 hits not feeling good about yourself. That's a very fixable situation. If you pitched 44 innings and gave up 80 hits, that's a different situation.' What we're talking about is a question of command."
Prior didn't appear to have full extension on his breaking pitches, Piniella said, but the pitcher wasn't holding back at all.
"I think this young man wants to get back to where he was," Piniella said. "Obviously, when you have success, you gain more confidence. He's struggled with success, so his confidence isn't quite where it should be. What do you do? You keep working and don't worry. I told him, 'When you leave the ballpark, forget the worrying. It doesn't do any good. Keep working, and let's get some smaller steps as opposed to trying to make that one big step.'"
Rothschild agreed it's too early to make a decision on whether the right-hander is ready for the regular season. The Cubs have four starters set, and a few pitchers competing for that fifth spot, including Angel Guzman, Wade Miller, Neal Cotts and even Sean Gallagher.
"If Mark goes back to throwing the ball the way we've seen in the past, the decision is fairly easy," Rothschild said.
Prior, whose best season was 2003, when he was 18-6, does feel as if he's made progress.
"I feel like I made some strides in some things, baby steps, nothing grandiose," he said. "Those are things you can build on. It's going to take me some time to get back to throwing the ball effectively. How much time? That's what you work for. You just keep taking little steps and trying to move in the right direction. One outing you feel it, and you're like, 'Darn, let's duplicate that.' That hasn't happened yet.
"You keep tinkering with some things and get to a point so it becomes second nature and it's not like you're fighting it," Prior said. "It's been awhile since it's been second nature, and that's what I'm trying to fight and get back to. Ultimately, it's about results and competing."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.