After settling down from the second inning on, Prior -- who went six innings on an 88-pitch count and gave up seven runs and nine hits with seven strikeouts and one walk -- said he was satisfied with his outing.
"I got knocked around. That's good," said Prior, who is scheduled for an April 12 start against the San Diego Padres. "Obviously the first inning wasn't what I was looking for, but I was glad to rebound and pitch well the last four innings. I started to get into a groove."
Prior said his location was poor during his April 2 rehab assignment against San Diego Padres minor leaguers because he was getting under the ball.
His goal Thursday was to place his fastball where he wanted it -- not to blow by guys.
The plan wasn't too promising in the first inning. The Isotopes' Mark Little led off the bottom of the first with a double off Prior's 92-mph fastbtall. The Isotopes then avalanched Prior with base hits, including first baseman Jason Stokes' two-run triple that traveled about 420 feet before bouncing off the wall in left-center field.
"I didn't make adjustments to hitters early," Prior said. "I left the ball over the middle (to Little) with two strikes. I got away from my aggressive fastball. After the first inning, I went inside more. I didn't give up the hard-hit balls after the first."
Prior soon found his location, throwing 35 pitches in the next three innings, 23 of which were strikes.
The Isotopes scored a run in the second off a throwing error by Iowa shortstop Ronny Cedeno. A sacrifice fly by Wilson Delgado made it 7-3 Isotopes in the fourth.
But Prior's command of the mound started to show -- 88 pitches through six innings is impressive, considering he said he started to attack hitters more as the game progressed.
"I felt stronger pitching through the course of the game," Prior said. "I got lazy with my mechanics at times, but overall my arm felt good. I had a good time out there."
His curveball, slider and changeup seemed effective for most of the outing. Five of his seven strikeouts came on two-strike offspeed pitches.
The fifth inning looked sloppy for Prior, who walked a batter and hit another with a pitch to lead off the frame.
The thought of an inflamed elbow frazzled Prior at times, he said, which prompted Iowa second baseman Richard Lewis to tell him to "not worry about it."
If Prior wasn't always sure of his stuff, some of the Isotopes were.
"I've never seen a curveball that sharp in my life," said Stokes, who Prior struck out twice. "He got me a couple of times. He's still one of the best."
Prior said his arm has felt 100 percent for the last 10 days, and that Thursday was another chance to hone his skills before April 12.
"That line has nothing to do with his performance," said Little, who knocked three hits off Prior. "He pitched well. I think he's definitely ready."
Prior's fastball never topped 93 mph, which he hit three times. Isotopes starter Travis Smith was clocked at 94 mph on Thursday.
Prior says he's eager to return to his form from 2003, when he went 18-6 with a 2.43 ERA.
Last year's Achilles tendon injury limited Prior to a 6-4 record. He posted a 4.02 ERA.
A fully confident Prior will reunite a Cubs rotation that's one of the best in the Majors.
"It's frustrating," Prior said. "You want to be with your team and help your ball club.
"But my arm feels good. I'm sure they'll ask me how I feel (when returning to Chicago), and I'm sure I'll say, 'I'm fine.'"