Boston University completed its first postseason assignment Saturday night, closing out its opening-round playoff series with a 5-4 win over UMass­ at Agganis Arena and advancing to the Hockey East quarterfinals.

The victory, coupled with Friday night’s 2-1 overtime win, set the Terriers up with a date next weekend against UMass-Lowell in another best-of-three series (games Friday, Saturday, and Sunday, if necessary) at Tsongas Center. It also sent UMass home for the season, the Minutemen finishing 2015-16 with a humbling 8-24-4 record after the near euphoria of a 6-2-1 start back in October.

“Proud of our guys finding a way to win . . . it wasn’t easy by any stretch of the imagination,’’ said Terriers coach David Quinn, after the ninth-ranked Terriers improved to 21-10-5. “A lot of that had to do with how hard UMass played. Some of it had to do with the way we played . . . taking penalties, a 10-minute misconduct [by Ahti Oksanen], which I think was a reflection of our mental commitment to the weekend.

“But I think when we needed to, we got it together, we played hard and played smart, and did enough to win.’’

Enough because Jakob Forsbacka Karlsson scored two goals, including one late in the third period that snapped the 4-4 tie and provided the winning margin. Enough because senior right winger Danny O’Regan, formerly of St. Sebastian’s, scored twice and added two assists to become the first Terrier since Chris Drury to crack the 150-point plateau (now 65-87 — 152). And enough because frosh blue liner Charlie McAvoy chipped in with a biggy-sized order of four assists.

But overall, the Terriers, hoping to return to the Frozen Four title game for a second straight season, too often were askew, particularly on defense. They surrendered leads of 1-0, 2-1, 3-2 and 4-3, be it because of weak, disheveled work around their own net or the compulsion to think offense when they should have been defense-first. Not a good cocktail when readying for the toughest part of the postseason.

“We’ve defended well in the last two months,’’ said Quinn, his displeasure clear. “This is as poor as we’ve defended. I thought we were flatfooted and on 50-50 pucks, we assumed offense. I think three of the goals were a direct result of that. The puck is going along the half wall and we’ve got people vacating the defensive zone . . . all of a sudden it’s in the back of our net. The good news is, we haven’t done that. So I am hoping it just happened this weekend, because if it continues, we are going to feel the pain in a hurry.’’

A sparse crowd of only 1,917 turned out at BU, where the student body was on spring break. For the two nights, the combined attendance was a paltry 3,669.

Unlike Friday night, the Terriers were first on the board in Game 2, Forsbacka Karlsson knocking in one lacrosse style at the right post on an O’Regan relay. Dennis Kravchenko neutralized that lead less than three minutes later when he roofed a short-range shot from the slot.

The two sides combined for five goals in a wild-west second period, the Terriers again quick to give up goals after taking leads of 2-1 and 3-2. But it was O’Regan, with his second strike of the period, who provided the 4-3 lead as the teams headed into the third period.

Back came the Minutemen, knotting it (4-4) for a fourth time at 3:51 of the third on Austin Plevy’s 10th goal of the season. And then it was left to Forsbacka Karlsson, the Bruins draft pick, to rip home the winner with 8:09 remaining in regulation. UMass managed only three more shots for the remainder of the night, BU goalie Sean Maguire (13-6-1) never seriously tested down the stretch.

“A lot of punch, counterpunch over the course of the 60 minutes,’’ said UMass coach John Micheletto. “Obvioiusly, they got the last one. But very proud of our guys’ approach, mentality and execution and mostly guts. It’s a man’s game out there and I thought we were ready for that challenge. I was an awfully proud coach to stand behind that bench tonight.’’

Kevin Paul Dupont can be reached at kevin.dupont@globe.com. Follow him on Twitter @GlobeKPD.