On Friday night, the Massachusetts hockey team took a giant step forward on defense with its second shutout win of the season.

But on Saturday night, the UMass defense took two steps backward in a 5-3 loss to No. 7 UMass Lowell at Mullins Center in front of an announced crowd of 3,678.

The trouble started right away for the Minutemen with two goals by the River Hawks in the first period. UML’s lead got as high as 4-0 early in the second period, leading to starting goaltender Steve Mastalerz being pulled.

“The difference in the game was the pace at which we played defensively,” UMass coach John Micheletto said. “We certainly had a decent jump offensively early. We didn’t close and take away time and space in the defensive zone, we didn’t straight line guys on the cycle and consequently we allowed them to have second touches right around our goaltender to put us down 2-0.”

Michael Fallon, who had three points, got things started with a goal off a rebound. A.J. White took the initial shot from the point, which bounced off of Mastalerz to a waiting Fallon in the right circle, and he finished from there.

UML increased its lead to 2-0 with only 29 seconds remaining in the first frame. The River Hawks initially got the puck into their offensive zone after a UMass turnover led to a breakaway chance. Mastalerz came up with the initial save but seconds later Adam Chapie found the back of the net for UML.

“It was certainly disappointing, frustrating, whatever the most appropriate word is,” Micheletto said of his defense’s inconsistency after the shutout last night.

The River Hawks went right back to work in the second period with two-straight goals and three total to take a commanding 5-2 lead.

The first goal was a textbook wrap-around goal for Michael Kapla two minutes, one second into the second period. Michael Colantone scored 51 seconds later on a shot from the point that beat Mastalerz blocker side.

Alex Wakaluk replaced Mastalerz (10 saves) in net after he gave up his fourth goal of the night.

Ryan McGrath scored the final River Hawks’ goal of the period off a rebound during a power play. That was the only puck that Wakaluk let past him as he made 18 saves in relief.

“I was happy for Alex Wakaluk coming in, in a tough spot, giving us an opportunity to get our legs back underneath us,” Micheletto said.

Ray Pigozzi gave UMass a little bit of life later in the second period when he scored off a pass from Adam Phillips at the 7:47 mark of the second frame to cut the lead to 4-1 at the time.

Conor Sheary got his 99th career point on a power-play goal late in the second frame to give the Minutemen a chance heading into the final frame.

UMass cut it to 5-3 at the 7:34 mark in the third period when Pigozzi fed Phillips, who beat UML goalie Connor Hellebuyck from the point.

The Minutemen had a great chance to cut down the deficit even more when they drew a power play midway through the final period. But UML, which has now won the last eight games against its in-state rival, successfully killed off the penalty without much threat from UMass.

“On the positive side, I thought our guys did a great job of battling back, wrestling the momentum back despite being down,” Micheletto said.

Phillips played alongside Michael Pereira and Pigozzi on the second line in Troy Power’s usual spot. Power injured the same left leg that caused him to miss most of last season when he suffered a knee-to-knee collision in the third period of UMass’ 3-0 win at Northeastern on Friday. Two teammates had to help Power off the ice after the collision in the neutral zone.

The team said that a team doctor evaluated Power on Saturday, but Micheletto said he didn’t have time to speak with the trainer about his status yet.

The Minutemen will be back in action next weekend with a road game at the River Hawks followed by a home game against Northeastern.

Cameron McDonough can be reached at [email protected] and followed on Twitter @Cam_McDonough.