Boston starter Jon Lester, penciled in a day early, gave up four earned runs in five innings last night against Los Angeles.

The Providence Journal / Bob Breidenbach

BOSTON — Hot pitching prospect Justin Masterson will make his big-league debut this afternoon at Fenway Park in the series finale against the Los Angeles Angels of Anaheim.

Masterson, the team’s second-round pick in 2006, will be promoted from Portland, the Sox’ Double-A farm club, to help plug a hole in the rotation caused by a nasty flu bug that has been seriously affecting the team.

The 23-year-old right-hander was 1-0 with a 0.95 earned-run average in four starts for Portland this season. The 6-foot-6, 250-pounder fanned 23 and walked only 5 in 19 innings.

Masterson’s emergency promotion has come as a result of health issues with Josh Beckett (stiff neck, flu) and Daisuke Matsuzaka (flu). Beckett was unable to make his start on Tuesday night, so David Pauley was plucked from Pawtucket for that outing. Dice-K was unable to make his start last night, so Jon Lester, today’s scheduled starter, was moved up a day.

To make room for Masterson, the Sox optioned right-handed reliever Craig Hansen after last night’s game. Hansen, recalled from Pawtucket earlier in the day, was the losing pitcher in Boston’s 6-4 setback to the Angels.

Boston manager Terry Francona said he was looking forward to seeing Masterson pitch in the big leagues.

“His future is very bright. Hopefully we’ll get a chance to get a glimpse of that [today],” said Francona.

Hansen’s stay is brief

About 3:30 yesterday afternoon, Jason Varitek was in the batter’s box at Fenway getting in some hitting for the first time in five days. In the clubhouse, Hansen had arrived and was dressing in a Boston uniform for the first time in two years.

The two situations were related.

Varitek has been hit hard by the flu bug that has run through the Red Sox. He has not played since Friday and his availability for last night had been in question. If he was still ill, it appeared likely the Sox would call up a catcher from Pawtucket, probably Dusty Brown.

However, when Varitek told club officials after Tuesday night’s game that he was feeling better, the Sox decided they did not need another catcher. The team had 13 pitchers on its roster Tuesday when it called up Pauley to make the start in place of Beckett. Since Varitek was feeling better, the Sox decided to stay with 13 pitchers. The bullpen was a bit short, in part because Manny Delcarmen also has been among those ill.

They promoted Hansen. It served a dual purpose.

For one, Hansen has been superb at Pawtucket. A first-round draft choice in 2005, he has not allowed a run in 12 1/3 innings covering eight appearances. He provided the Sox with a ready arm in the bullpen.

Beyond that, it gave Hansen, who had pitched with the Sox for parts of his first two seasons, a chance to revisit the bigs, even if it might be for only one day.

The sick and wounded

Francona sounded more like a doctor than a manager in his pregame meeting with the media. Most of the discussion centered on injuries and illnesses –– and that was before anyone knew Matsuzaka was sick, too.

Here are some of the topics:

Kevin Youkilis was given a night off because of a minor problem with his back. He felt some discomfort in his lower back when he went deep behind third base to make a play in Tuesday night’s game.

“He called me this morning and told me he was feeling it,” Francona said.

Jed Lowrie started in his place and hit eighth. Julio Lugo moved up to the seven hole and Kevin Cash hit ninth.

If all goes well, Varitek will return to work for this afternoon’s series finale against the Angels, although that is not definite, Francona said.

Beckett is feeling better and has been penciled in to pitch Sunday, which would be his normal spot in the rotation.

Mike Lowell not only took more batting practice yesterday, but did some work in the field for the first time since going on the disabled list. Depending on how he feels after the work, he could be on rehab assignment soon.

Coco Crisp is feeling better and probably will return to the starting lineup today.

Extra innings

Lugo’s RBI single in the second extended his hitting streak to nine games. He was one of seven players in the Sox’ lineup who began the night hitting over .300.

skrasner@projo.com