But Wrigley Field quickly snapped back to reality as another ninth-inning lead slipped away in a 2-1 loss to the Pirates.
Prior retired the first 13 batters he faced and allowed only two hits, striking out eight and walking none while throwing 85 pitches.
Considering the magnitude of the event, which had been widely anticipated since his Achilles' tendinitis became public in late February, Prior had a phenomenal outing.
"I had some anxiety going," Prior said. "I was probably a little nervous. I didn't have a great bullpen [session] warming up, but obviously that doesn't mean a whole lot. There are still some things I need to work on, but for the most part it was pretty good."
Prior left with the game a scoreless tie.
Closer Joe Borowski couldn't protect a 1-0 lead, giving up two runs in the ninth and jeopardizing his job with his second blown save in a week.
Baker declined to give Borowski his usual vote of confidence after the game.
"I'll think about it and make a decision," Baker said.
Borowski, now 2-4 with an 8.02 earned-run average, said whatever decision Baker makes is fine with him.
"Sooner or later you're going to get those breaks and turn it around and hopefully run from there," Borowski said. "It just seems like I'm pretty much stuck in a bad dream, and it's tough to get out of it."
The Cubs stranded 11 runners, leaving the bases loaded in the seventh and failing to get the tying run home from third with one out in the ninth.
Borowski replaced LaTroy Hawkins to start the ninth after the Cubs had grabbed a 1-0 lead in the eighth on Todd Hollandsworth's RBI single.
Moises Alou made a fine diving catch to rob Jason Kendall for the first out.
After Jack Wilson doubled to the wall in left, Borowski induced Daryle Ward to pop to third.
But Craig Wilson singled through the hole at short, bringing home the tying run.
With left-handed-hitting Rob Mackowiak coming up, Baker made the rare move of replacing Borowski with Mike Remlinger.
Mackowiak had hit a game-winning grand slam off Borowski a week ago in Pittsburgh.
"I pretty much gave up the lead right there, and you have a lefty coming up who's pretty much torched us this whole year," Borowski said. "So he's going with the matchups there."
But Pittsburgh manager Lloyd McClendon pinch-hit Chris Stynes for Mackowiak, and Stynes singled to right to bring home the go-ahead run.
The Cubs threatened in the ninth off Pirates closer Jose Mesa when Jose Macias doubled and Todd Walker sacrificed him to third.
But Derrek Lee hit a foul pop to first and Corey Patterson flied to center to end it.
"In that situation, you have to get that run in from third base," Baker said.
Prior's superb outing was encouraging news for the Cubs, but Baker's dilemma over the closer's role has taken over as the crisis of the moment.
"Nobody ever said baseball is an easy game," Borowski said. "Whatever [Baker] decides is whatever he decides. I'm not going to worry about what might happen or what can happen or anything like that. I can't worry about things I can't control. Whatever is best for this team at this time is fine.
"Dusty is the manager, and he has the right to do whatever he wants. I'll respect what he does. Hopefully he'll have that belief in me and stick me out there next time."