Islanders defenseman Matthew Schaefer follows through on a shot against...

Islanders defenseman Matthew Schaefer follows through on a shot against Chicago during the third period at UBS Arena on Thursday in Elmont. Credit: Brad Penner

Pete DeBoer has every reason to allot heavy ice time to Matthew Schaefer, especially now with Alexander Romanov out for the season. But having the 19-year-old defenseman, already one of the NHL’s top players, fresh and effective for the late-season push is crucial to the team’s playoff chances.

What’s a coach to do?

“It’s hard not to play those guys,” DeBoer said before the Islanders beat Chicago, 4-1, at UBS Arena on Thursday night when he was asked about balancing in-game needs with the full-season big picture. “It’s hard to keep those guys off the ice and it’s particularly hard when you’re losing games because you know those guys are the difference-makers.

“But we’ve been very conscious from Day 1. We’ve taken his penalty-killing away and used other guys.”

Schaefer still logged 26:43 with a power-play assist against struggling Chicago, missing its best player in Connor Bedard.

The game turned chippy in its waning minutes after Chicago’s Frank Nazar got his shoulder up high on Bo Horvat, leading to a match penalty and a 10-minute misconduct and sending Horvat, who had two goals, into concussion protocol. No immediate update was available.

“They go after one of our guys,” Emil Heineman said. “Whatever it is, we have to stick up for one of ours, and I think we did that. It’s a cheap shot. I don’t like to see that. I like the way we responded.”

Sticking up for teammates is one of DeBoer’s “non-negotiables.”

The coach faces almost an internal negotiation each game about how much to use Schaefer. It’s a delicate dance, and Thursday’s game provided the first glimpse into exactly how Romanov’s second straight season-ending injury — this time he needs surgery to repair the ACL in his right knee after battling back from shoulder surgery — will impact Schaefer’s minutes.

Schaefer, who remained on his off-side despite the lefthanded Romanov’s injury, skated with fellow lefthanded shot Logan Stanley as DeBoer was forced to reconfigure his defense pairs. He reunited Adam Pelech and Ryan Pulock and moved righthanded Tony DeAngelo to his off-side with fellow righty Scott Mayfield, who made his season debut after being a healthy scratch for the first three games.

Stanley, signed as a free agent on Sept. 27, had two assists in 23:49, the first time he’s logged 20-plus minutes as an Islander.

Playing with Schaefer this quickly into the season was not something he was anticipating.

“He likes the puck on his stick,” Stanley said. “He’s a world-class skater. There’s very few that can skate like him in this game. Give him the puck and let him do his thing and try to be solid defensively for him.”

Schaefer averaged a team-high 24:27 of ice time through the first three games, including 25:03 in Tuesday night’s 5-2 loss to the Rangers at Madison Square Garden, with Romanov exiting in the third period after a collision with Mika Zibanejad.

But DeBoer used Schaefer, who plays on the top defense pair and quarterbacks the first power-play unit, for a judicious 20:34 in Saturday’s 6-0 win over the visiting Devils. That suggests that DeBoer — who took over for Patrick Roy with four games remaining last season — is very cognizant of Schaefer’s ice time and is trying to manage it appropriately.

Last season, Schaefer led the Islanders with an average ice time of 24:41, setting a career high with 31:59 in a gut-wrenching 4-3 loss to Chicago at UBS Arena on March 24.

So what, if anything, fatigues Schaefer?

“It’s a good question,” he told Newsday. “When you’re in the game, adrenaline, you’re like, ‘I don’t care if I’m tired, I’m going to go out there and shut off that “I’m tired’’ mentality.’ There can be times where it’s a very long shift and I’m a little gassed, but I try not to show it.”

Schaefer added that he recovers quickly on the bench after a tiring shift. The joys of being 19.

The reason it’s so attractive for DeBoer to over-rely on Schaefer is simply because the Islanders look so different — so much more dangerous — with him on the ice. Slight hyperbole, but against the Rangers, it really seemed the only time the Islanders could effectively break out of their own zone was during Schaefer’s shifts.

Still, the Islanders should have fewer issues getting up ice whenever top-six forward Mathew Barzal (left knee) is able to return after missing training camp. Barzal began skating on his own Thursday.

“He’s a key player, and having him up front will help us all around,” Schaefer said. “He’s been battling something, so for him to start feeling better and get close to 100%, it’s going to be awesome. Seeing him skating is awesome.”

DeBoer’s inclination is to have Schaefer skate heavy minutes frequently during a game.

He will have to temper that urge as best he can.

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