BOSTON — Just as they’ve finally had their day, their days are almost certainly numbered.

Matt Grzelcyk and Brandon Carlo are in their third and fourth seasons on the Bruins defensive corps, a group that has always seemed to be without one or two full-timers, and often more. The situation reached ridiculous proportions last year, when the B’s needed 12 blue-liners to get through 2018-19.

Yet Grzelcyk, a left shot, and Carlo, a righty, never ended up paired together until this season, when the defense corps has sustained exactly one injury.

“It is kind of strange when you think about it,” Carlo said, “but if (Grzelcyk) and I ever played together, it was for maybe a period or two, but not for a full game.”

Carlo and Grzelcyk will team together for the fourth straight game on Tuesday night at New Jersey (7:05, NESN, WBZ-FM 98.5), and it may be for the last time. Torey Krug, Carlo’s usual partner on the Bruins’ No. 2 pairing, has recovered enough from his upper body injury to start skating, and could be available to play later this week.

It has been a learning experience for both players, but a bit more of an education for Grzelcyk. While Carlo has been a first- or-second pairing defender since entering the NHL with virtually no minor league seasoning in 2016-17, Grzelcyk has played primarily on the third pair since battling onto the roster in 2017-18, after a season-plus at AHL Providence. The No. 2 pairing generally skates against top-six forwards, who play more than third- and fourth-liners, and Grzelcyk has also assumed Krug’s duties with the Bruins’ top power-play group.

“You do see certain things,” Grzelcyk said. “You notice the consistency of going out against the other team’s first or second line almost every time. You do get more minutes, so you feel that in back-to-back games.”

Grzelcyk, whose ice-time average for the season is 18 minutes, 32 seconds per game, has played more since moving up to join Carlo — sometimes significantly more: He played a season-high 23 minutes, 8 seconds last Tuesday against Florida, and was on the ice for 22:52 on Saturday against the Capitals after playing 19:30 a night earlier at Toronto. The games against the Panthers and Caps both went to overtime.

Grzelcyk said he has noticed the bump in playing time “kind of in the middle of the game. Maybe you’ve had a few more shifts than usual, so you’re into the game, and your motor’s going. You might be a little more tired physically, but I also think it gets you into the game a little bit more.”

Carlo, who played as a rookie with B’s captain Zdeno Chara in 2016-17, gave way to Charlie McAvoy a year later and began to establish a duo with Krug in ‘17-18. He has enjoyed the opportunity, however brief, to get to know a teammate as a partner.

AROUND THE BOARDS: Center Patrice Bergeron, who missed Saturday’s 3-2, shootout loss to the Capitals with a lower body injury, traveled with the team to New Jersey and will test his injury on Tuesday morning. Cassidy said Bergeron’s availability is a game-time decision ... Wingers Jake DeBrusk (lower body) and Brett Ritchie (upper body), who have both missed the last five games, went through Monday’s full practice; Cassidy said “we anticipate they’ll play.”

... Forward Brendan Gaunce was recalled on Monday from AHL Providence, likely as insurance if two of the three injured forwards on the trip don’t play.