FORT MYERS, Fla. -- Under the baking sun at the Minor League complex on Thursday, Red Sox right-hander Justin Masterson -- the Swiss army knife of the Boston pitching staff -- again had his starters' hat on. The sinkerballer came pretty close to getting all the way stretched out, throwing 81 pitches over 4 2/3 innings against Cincinnati Reds Minor Leaguers.

So now what?

More intrigue, that's what.

Masterson will throw a side session on Saturday -- the same day Brad Penny makes his second Grapefruit League start.

Beyond that?

"To be announced," said Red Sox pitching coach John Farrell.

At the moment, Masterson and Penny are tied closely together. Can Penny be ready in time to start the season in the rotation? If not, Masterson will likely get the call on April 12 at Anaheim -- when the Red Sox first need a fifth starter. Clay Buchholz, who has had a superb Spring Training, is also in that mix.

"Not only [Penny's] health, but will he be ready for that start on the 12th, or do we take a more conservative approach that would create one or two starts for a fifth starter?" said Farrell. "Certainly Clay Buchholz is in that mix as well. Any final decision or determination will certainly come from [general manager] Theo [Epstein] or [manager Terry Francona], but that's part of the information gathering at this point for us."

Though Farrell, Francona and Epstein all spent what was an off-day for the Red Sox watching Masterson face Minor Leaguers, there isn't a whole lot more information they need to gather about him. Masterson proved last year in emphatic fashion that he is a solid Major League pitcher, no questions asked. His versatility is not complicating, but welcome.

"That's the coolest thing, that's the neatest thing," Masterson said. "We're standing here talking about whether or not I'm going to be starting or relieving and never has there been a mention of, 'Oh, am I going to be in the big leagues or Triple-A?' That's really cool that there's a spot, and it gives me some confidence as we go out here to just continue to get better even as we're continuing to get higher pitch counts and more innings."

Masterson had been a starter for basically his whole life until the Red Sox converted him to a reliever for the second half of last season, and he responded by being dominant both in the regular season and postseason.

Thursday's performance -- in which he gave up four hits and no runs while walking two and striking out six -- was his first truly lengthy performance since July 5 of last season, when he went six innings at Yankee Stadium.

"It was pretty enjoyable," Masterson said. "Warm day, felt good out here. It was fun to be out in the sun. Yeah, I was happy with the day. It's been a while since I've gone 4 2/3 innings, whatever it was.

"There is an oddness to, 'I'm going back out again, going back out again, going back out again', but I think it's nice," Masterson said. "Body-wise, I felt pretty good, even when I came to the end of the pitch count, so I'm excited to see what will still happen as we continue to evaluate."

The stretching-out process of this spring has allowed Masterson to become more of a pitcher again. He's had ample time to bring his changeup and slider back into his pitch mix.

"My slider feels really good," Masterson said. "My changeup -- overall, I feel good. There were about three or four today where I just took the whole body and left the arm behind. I did that on some sliders, too. But overall I'm really excited about what hopefully the stuff will be. I have confidence in the changeup and the slider, and hopefully I can carry that to the season."

If Masterson should get the call against the Angels, he clearly feels up to the task.

"I'd love to think so," Masterson said. "I feel good. Everything is coming out of the hand well, aside from the few times where I just want to get under the ball and not do what I'm supposed to do. Aside from that, I feel strong and I think I'll be able to help the team out for five, six, seven -- whatever we need."

And he added one more thing.

"As long as everything is put together and we can have a winning season this year, I'm going to be happy," Masterson said.