"I felt really, really, really good today," said Papelbon, who was charged with the loss in Cincinnati's 2-1 victory at City of Palms Park.
Papelbon went 3 2/3 innings, allowing two runs on five hits, one walk, five strikeouts and one hit batter.
"My body felt outstanding. My arm felt outstanding. Everything physically felt really good today, and that's a big, big plus for me," he said. "[I'm] getting my body into that stamina that I need to and get everything just built up to becoming a guy that can go seven, eight innings. So it was huge for me to feel the way I felt and how good I felt."
In his first two Spring Training appearances, Papelbon (0-1, 2.08 ERA) followed starter Tim Wakefield to the mound, building up his innings and pitch count as he prepares to transition from closer to starter.
Before the game, Sox manager Terry Francona said Papelbon would go four innings and about 65 pitches. Papelbon threw 72 pitches (42 for strikes), allowing Francona to bring Joel Pineiro in, as planned, in the middle of an inning.
"[Papelbon looked] pretty strong," Francona said. "I actually thought today at the end of the game [he had] one of the best fastballs he threw. I think he agrees. The umpire actually stopped me and said the same thing. [Pitching coach] John Farrell and I were discussing that in the last inning. It seemed like he got into a little bit of a groove with a different catcher for probably the first time. I thought he was trying to pick a little bit early. But I thought he was fine, looks healthy, looks strong, and I thought he got stronger, which is good."
Papelbon admitted to being selective early, working with his two-seam fastball as he relearns the starter's role, and taking advantage of facing a team he won't see in the regular season to work on pitches and situations.
"I was definitely working on my two-seam a lot today," he said. "I think with a ballclub like Cincinnati -- we don't play as much -- is a perfect opportunity to do that. I just felt like I was trying to nitpick with it a little bit too much instead of just pounding the strike zone and getting ahead of hitters."
Of the 17 batters Papelbon faced, he fell behind in the count to at least seven, before striking out his last batter, Juan Castro, swinging on a 1-2 fastball.
"For me, it's going to be important to get that game started off on a good foot those first couple innings, and establish the strike zone and pounding the strike zone," Papelbon said. "But that's a learning experience for me. Now that I know that, I can expect that the next start."
Francona is not concerned about Papelbon, who last started a Major League game on Aug. 21, 2005, a 5-2 win over the Angels, as he makes the transition back to being a starter.
"I think pitchers, as long as they know how to get outs, I think they're OK," said Francona. "He's done this a lot. He's relieved and started. He's OK. Good pitchers, they can just pitch. I thought he threw some good breaking balls today, too, probably one of the better ones we've seen all spring."
Papelbon was able to work in all his pitches Saturday, something he did not have to do last year, when he relied primarily on his four-seam fastball and splitter as a closer.
"The fact that I threw my curveball for strikes, threw my slider for strikes and threw my slider for out pitches [was another big positive out of my outing today]," he said. "My split felt really good today as well. The only thing that I really am not pleased about is that I didn't really pound the zone and get ahead of guys. I felt like I could have done that a lot better."
Papelbon said he found himself falling into a familiar role at times during the game, as the closer persona in him emerged.
"When I started falling behind hitters and I started getting a couple guys on with no outs and walked a guy and they were getting on, it was like I had to go back to that closer mode in my mind and say, 'All right, I got men on. I got to get some strikeouts and then maybe a ground-ball double play,'" he said. "It was real weird for me, because my whole mind-set changed. ... I think that's going to work to my advantage, because I did come into a lot of situations [last season] with guys on and nobody out, and I was able to make my adjustments and get out of them. So I'm going to use a little bit of that closer mentality when I need to, but at the same time go about my business as a starter and try and get the ballgame under way and get my team back in the dugout."
With Wakefield, who pitched in a Minor League game Saturday, and Papelbon on the same starting schedule, Francona is not sure yet when Papelbon will start next. The team's only scheduled off-day of the spring is Monday, which will likely factor into the decision.
Maureen Mullen is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.