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5/24/2014 9:43 P.M. ET

Mets still looking for perfect setup in bullpen

NEW YORK -- While Mets manager Terry Collins has yet to name Jenrry Mejia his official closer, the 24-year-old righty is getting every opportunity to claim that role indefinitely. That makes the biggest question in the bullpen the eighth-inning man -- or men.

Collins noted that while there is some benefit to having a lack of clear bullpen roles -- it keeps the relievers on their toes -- he would like to have "that one guy" for the eighth inning.

"Right now, we haven't settled on that," he said.

There does not appear to be one obvious choice.

Daisuke Matsuzaka pitched a scoreless eighth when the Mets beat the Dodgers, 5-3, on Thursday, but he's getting a spot start in Sunday's doubleheader and is still adjusting to becoming a reliever. Jose Valverde, the Mets' one-time closer this season, has seen the eighth five times.

Scott Rice owns a 0.00 eighth-inning ERA, but he has been more of a lefty specialist. His 13 appearances in the penultimate frame have spanned just 5 1/3 innings. Nine of Carlos Torres' 24 games have included eighth-inning work.

Whoever ends up settling into that role, Rice agrees with Collins: Better to have it be one man, rather than by-committee.

"Knowing when you're going to get into a game obviously always helps. You prepare yourself and go through your routine to get ready," Rice said. "Sometimes you'll be sitting there and you won't know. You'll have to be mentally locked in from the fifth inning to the eighth inning, ninth inning, whenever it is. Over the course of the season, it can wear on you a little bit."

Added Collins: "It goes back to the game today, and the game today is roles. Everybody has to have a specific role. They get themselves prepared better. When they walk through the door they [think], 'Hey, I'm pitching the eighth, that's my job.'"

d'Arnaud to begin rehab assignment Sunday

NEW YORK -- In one of the last steps before returning from a concussion, Travis d'Arnaud will begin a rehab assignment on Sunday with Double-A Binghamton. He is expected to start behind the plate for the team's 6:35 p.m. ET game against New Britain.

On Saturday, d'Arnaud ran and took batting practice outside. Manager Terry Collins expected him to run the bases as well.

"This is the final stages of what he's got to do," Collins said. "I know he's going to be fine. He felt great yesterday with everything he did."

Collins suggested previously that d'Arnaud's rehab assignment could be as short as eight to 10 at-bats.

d'Arnaud has been out since May 13, when Alfonso Soriano's backswing caught him in the back of the head.

Black will be back as doubleheader's 26th man

NEW YORK -- Vic Black is back -- almost.

The right-handed reliever will join the Mets on Sunday as the 26th man on the roster for the team's doubleheader with the D-backs, following Friday's rainout. It will be Black's second Major League stint with the Mets since coming to New York in a trade with Pittsburgh last August.

Black struck out 12 and walked four while allowing five earned runs in 13 innings last September, a month that gave the organization a glimpse of a pitcher who is seeping in talent, but has struggled with command. Black, 26, has walked about one batter every other inning in six Minor League seasons.

Those struggles continued this spring with Triple-A Las Vegas, where a 1.55 WHIP accompanies Black's 1.45 ERA. More batters have reached via walk (18) than base hit (12) in Black's 18 2/3 innings.

"He has not been getting hit, but the command has not been what you would like it to be in a guy you project to be the closer one day," said Mets manager Terry Collins. "So if we can get him to have better command of his stuff, he'll never see the Minor Leagues again."

Collins, who did not rule out the possibility of Black staying in the Majors beyond Sunday, raved about the reliever's arsenal of pitches. Last year, when he pitched in 18 Major League games for the Pirates and Mets, Black relied mostly on a fastball that averaged 96 mph and a curveball that dipped into the low 80s.

"It's all there," Collins said. "He's just got to get it in the zone."

Worth noting

Wilmer Flores' throwing error, which came in the third inning of the D-backs' 3-2 win on Saturday, received plenty of attention after the runner, Paul Goldschmidt, came around as the eventual winning run. Flores fielded a grounder deep in the shortstop hole and, after failing to get a good grip on the ball, bounced one to first baseman Lucas Duda.

"I didn't [get a good grip]," Flores said. "I just made a bad throw."

• Right-handed reliever Gonzalez Germen is not close to returning. He was placed on the 15-day disabled list with flu-like symptoms, but has since developed an abscess.

"I don't necessarily know if I'm the guy to tell you where it is. It's not in his shoulder, I can assure you of that," Collins said. "You probably don't [want to know]."

Tim Healey is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.