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NEW YORK (AP) — Ryota Igarashi is back in the New York Mets' bullpen — and Jenrry Mejia is staying there, too.

New York optioned right-hander Manny Acosta to Triple-A Buffalo when Igarashi was activated from the 15-day disabled list Sunday, keeping the 20-year-old Mejia in a big league relief role.

In other news, second baseman Luis Castillo was scratched from the lineup Sunday night against the New York Yankees because of a sore left foot that's been bothering him for a while. He was replaced by Alex Cora, who hit a two-run single in a four-run second inning that sent the Mets to a 6-4 victory.

Igarashi immediately will be thrust into an eighth-inning setup job. The rookie right-hander from Japan had a 1.35 ERA in seven relief appearances before going on the disabled list April 21 with a strained left hamstring.

"It's very important for us if he can take on that position," manager Jerry Manuel said. "It'll be very critical in going forward that he handles that role for us. It's been somewhat troubling for us lately, that eighth inning."

Igarashi entered in the ninth inning with a 6-1 lead Sunday night and struggled. He managed only one out, allowing three runs and two hits with one walk. Francisco Rodriguez came in for a save.

The decision to demote Acosta means the Mets will keep Mejia, a touted right-hander, in the majors as a reliever rather than send him to the minors for seasoning as a starter, his role prior to this year.

"Basically, that was the plan all along, if we felt that Igarashi can handle the eighth and we needed someone to handle the seventh," Manuel said.

Acosta was 1-1 with a 3.00 ERA in 12 relief outings for the Mets. He struck out 14 and walked eight in 12 innings.

Mejia is 0-2 with a 3.60 ERA in 21 relief appearances.

"In Washington when we were quote-unquote stretching him out, I think he lost his delivery a little bit and didn't get back, didn't correct that. So basically what that was telling me was that here at least at this point in his career, you have a young man who has a tremendous arm, a tremendous upside, that if you take what he gives you with that in small bits, it's very serviceable, even at the big league level," Manuel said. "Here, if it's serviceable and it's usable, then I'd like to have it. It's probably selfish on my part, but that's how I feel."

Also, knuckleballer R.A. Dickey was chosen to start in place of injured John Maine on Tuesday night against Philadelphia. Hisanori Takahashi goes Wednesday and Mike Pelfrey on Thursday.

In a quirky twist, Dickey will be the second consecutive knuckleballer to face the hard-hitting Phillies. Boston's Tim Wakefield tossed eight scoreless innings against them for an 8-3 victory Sunday.

"I don't think it has much to do with my outing," Dickey said. "It's good that he threw a shutout instead of giving up 15, I'll tell you that."

Asked if he thought Wakefield's performance against Philadelphia was a positive sign for Dickey, Manuel said: "I don't know. I couldn't hit him anyway. I wouldn't have any idea with that."

"R.A. throws a little harder than Wakefield. Whether that makes a lot of difference or not, I don't know," the manager added.

Dickey said he's been tutored by Wakefield in the past and he planned to call him to pick his brain again Sunday night or Monday.

"It's a small fraternity, so we're pretty tight," Dickey said.

With Maine on the disabled list, New York will need a starter Saturday at Milwaukee. Manuel said left-handed reliever Raul Valdes is an option, or the team could bring up someone from the minors.

Elsewhere, infielder Daniel Murphy, sidelined all season by a sprained right knee, went 5 for 5 with a double and three RBIs for Class-A St. Lucie against Lakeland. He played the entire game at first base and is batting .727 in three rehab games.