BOSTON — In two spot starts for the Red Sox, Justin Masterson has pitched a combined 12 1/3 innings and given up just two runs. Not bad, especially for someone who otherwise has not competed above Double A.

But there’s a good chance that when Masterson is next in a Red Sox uniform, it will be as a reliever and not as a starter.

As expected, the Sox optioned out Masterson immediately after last night’s game, a 2-1 win over Kansas City. Tellingly, they sent him to Pawtucket, and not Portland. Then again, Masterson had little else to prove at the Eastern League level.

For now, Masterson will slide into the PawSox rotation and continue to amass innings and experience. Unless there is an onslaught of injuries to the Boston rotation, even as well as Masterson has pitched here, there won’t be an opening in the major-league rotation in 2008.

The bullpen? That’s another matter.

While the Red Sox still envision Masterson as a starter, thanks to his size, durability and assortment of pitches, his quickest path to an extended stay in Boston would be in a bullpen role.

More and more, teams are introducing prospective starters as relievers. The Florida Marlins are doing it with Andrew Miller, a former No. 1 pick and a key part of the deal that sent Miguel Cabrera and Dontrelle Willis to Detroit last winter.

But the model here is Joba Chamberlain, whom the Yankees –– led by Hank Steinbrenner –– still view as a starter, but merely saved the season last year with his super set-up work in the eighth inning and remains there since the Yanks lack another trustworthy option.

Masterson’s stuff may not be not on a par with Chamberlain’s –– to be fair, few could claim that distinction – but it’s plenty good enough, and for the purposes of this role, would only get better.

Instead of pacing himself to go through a lineup three times, Masterson could channel his energy for three outs. His velocity, already 91-92 mph, would only improve under such conditions.

Finally, there is the matter of workload. The Sox zealously monitor the number of innings their pitching prospects work, not wanting to overburden them. By allowing Masterson to continue to start until, say, the All-Star break, the team could ensure he doesn’t go over his prescribed limit by switching him to relief.

“Could he do it?” said a veteran Red Sox player, pondering the question for a while. “Yeah, I don’t see why not.”

And here’s the thing: The Sox may have a real need for the final two months. Already, the bullpen appears suspect beyond closer Jonathan Papelbon.

After three straight scoreless appearances, Manny Delcarmen has failed to post clean outings in his last two appearances. Three years after being drafted, Craig Hansen remains very much a work in progress. And following five days off, Hideki Okajima returned to the mound last night and needed 28 pitches to get two outs in the eighth inning.

This much is certain: What little bullpen help is available at the trade deadline will be cost-prohibitive. Further, as the Sox learned to their dismay last season, obtaining a veteran closer with the idea of transitioning him into an unfamiliar set-up role has its own risks.

Masterson has already had some exposure to the role, pitching in relief one summer in the Cape League before returning to San Diego State for his junior season. The experiment went smoothly.

“Whatever they want me to do, I’ll do,” said Masterson. “The strength of the staff here is the starting rotation, so I guess it would make some sense. It would really just be a matter of coming in and letting it loose and pounding the zone. I don’t think it would really too hard.

“It’s all about mindset. You just say, ‘I’ve only got one (inning) –– let’s go after it.’ ”

It wouldn’t take long for Masterson to prepare for the new assignment. While ramping up and adding innings must be a careful progression, the reverse isn’t true.

“I’d have to get my arm used to back-to-back appearances, those kind of things,” he said. “Hopefully, I could be ready in a couple of weeks. And if we don’t have that much time, I could do it sooner.”

Masterson sounds eager and willing. It will be up to the Red Sox to determine if and when he’s ready.

smcadam@projo.com