TRENTON — Replacing David Phelps is not going to be an easy task by any means, but that’s exactly what the Thunder’s relievers were asked to do last night after their team’s ace was promoted to Triple-A Scranton/Wilkes-Barre. The bullpen held down the Harrisburg Senators all night long in a 4-1 win in front of 4,218 at Waterfront Park.

“It always comes down to pitching,” Franklin said. “We pitched well tonight but we didn’t hit well tonight, but we’re going to be in some close games. The two years that we won championships it was the pitching, and it always comes down to pitching.”

With the win, the Thunder gain a game on New Hampshire, who were blanked last night by the New Britain Rock Cats.

Possible candidates to replace Phelps long-term include George Kontos, who spent parts of two seasons in the Thunder rotation, and Adam Warren, the Yankees’ fourth-round selection in the 2009 draft.

Kontos, who is recovering from Tommy John surgery, has accrued a 3.12 ERA over four appearances — two of which were starts — with High-A Tampa. Warren is 7-5 with a 2.61 ERA and 51 strikeouts in 69 innings in his first full season as a professional.

Wilkin De La Rosa or Ryan Pope, both of whom have started this season, could fill the hole, but manager Tony Franklin indicated before the game that Phelps’ replacement would likely come from outside the current roster.

Cory Arbiso started and limited the Senators to a run on six hits over the first four innings before sitting down for the night.

J.B. Cox entered in the fifth and looked much better than his ghastly 8.59 ERA over five games with High-A Tampa. Using a sinking fastball that sat between 85 and 89 miles per hour, the former second-round pick silenced the Senators. He allowed a hit per inning.

Because Arbiso didn’t go the requisite five innings, the official scorer awarded Cox the victory. It was his first professional win since Aug. 14, 2008, against the Lehigh Valley IronPigs. It was first Double-A victory since August 6, 2006, against the Bowie Baysox.

Before the game, Cox, who spent a year away from the game, said it’s been long ride from his low point until last night.

“It’s been really tough,” Cox said. “It’s been an emotional and physical roller-coaster.”

In the seventh, Cox handed the ball to Kevin Whelan, who worked a pair of scoreless innings with two strikeouts.

A curious ball-strike call in the first inning helped turn the inning in Trenton’s favor.

With two outs and a 1-2 count on Austin Romine, Harrisburg starter and former Washington National Ross Detwiler pumped in a fastball that appeared to catch the inside corner. The pitch was so juicy, in fact, that Romine and catcher Jhonatan Soltano both turned and headed to their respective dugouts.

Plate umpire Jon Saphire, however, agreed with neither man and ruled otherwise, audibly calling the pitch inside.

Romine took advantage and worked a walk. Brandon Laird followed suit, and Jose Gil cashed in with a sharp double under the glove of first baseman Chris Marrero that brought Romine home.

The Thunder upped their advantage in the second inning on a two-run double from leadoff man Dan Brewer.

Edwar Gonzalez connected on a solo home run in the eighth, and Ryan Pope worked the ninth to seal the game and earn his second save.