The Giants’ ever-changing quarterback situation is testing offensive coordinator Matt Nagy’s math skills. 

Nagy estimated that the Giants lost about 30 percent of their playbook — namely, the dual-threat quarterback section of zone reads and RPOs — when Jaxson Dart was injured in Week 2 and replaced by pocket passer Jameis Winston for four starts. 

The latest variable in the equation is the introduction of J.J. McCarthy, who leapfrogged Winston to get the next start Sunday against the Texans. 

“Maybe now 20 percent of that 30 that was gone is back,” Nagy said before Thursday’s practice. “For us, what does he do well? You’ve seen some of the packages he’s done, but that can become predictable. We don’t want to be predictable. Now that he’s the starter, it probably benefits us some that he’s just not always coming in to do predictable QB-driven runs.” 

Making just his second appearance since he was acquired in a Sept. 28 trade from the Vikings, McCarthy produced a historic fourth quarter — 243 passing yards and three scoring drives — off the bench last Sunday in nearly bringing the Giants back from a 24-point deficit in a 34-27 loss to the Saints. 

The performance backed up the coaching staff’s belief that McCarthy turned a corner last week and was ready for a bigger role. 

“Just the confidence of him being able to, in practice, talk to us about the scheme, where his eyes are going in the pass game, where he’s snapping the ball with a motion or a shift,” Nagy said. 

“If there was a shoestring with Jameis or he got hurt, he could go in and function. Whereas the first two weeks, I had a highlighted [play] call sheet that, if something happened and he was in, these were the plays. It was probably 50 percent. Last week, it was probably 90 percent. Now this week, it’s going to be 100 percent.” 

Tight end Isaiah Likely said it is the responsibility of the other 10 offensive players to make McCarthy “as comfortable as can be [for] a quarterback in a new offense.” 

McCarthy has at least two pass catchers who believe they are always open. So, he’ll throw his 50 mph fastballs — among the fastest quarterback velocity in the league — downfield (12.4 career yards per completion) against a notoriously ball-hawking secondary. 

“Letting him know that as long as the ball’s in the air and in my vicinity, it’s a great pass either way,” Likely said. “It doesn’t matter if somebody’s on me or if I’m wide open. Just throw the ball, trust me, and I’m going to make the play for you.” 

McCarthy is the eighth quarterback that Malik Nabers has caught a pass from in 25 career games. His strategy for adapting to quarterback change is simple. 

“Get open; just get open,” Nabers said. “They’re going to find you, and they know who their guys are. And when you talk to them, just give motivation to them. Tell them, ‘I know you’re just coming in, you don’t know me, really … but just know on certain plays, just give me an opportunity. Even when I’m covered, just give me an opportunity.’ ” 

McCarthy aced the no-huddle part of the playbook against the Saints. But what about managing a four-quarter game? 

“His operation is really, really good,” Nagy said. “It starts with listening to the play call in your headset, being able to say it and/or read it on the wristband, get to the line of scrimmage, and then be a quarterback: see the formation, motion shifts, see the play clock, make your ID checks or whatever you do with the center, and then make a play. What he’s doing is amazing. He wants as much as he can get to digest within our offense.” 

The Giants could have not thrown McCarthy into a hostile road environment as 9.5-point underdogs and given him two weeks to prepare to make his first start after the bye. Why wait? 

“We would not have had the discussion if we didn’t think he could do it,” Nagy said. “We fully feel as a staff [quarterbacks] coach [Brian] Callahan has done an amazing job working with him and getting him ready. We feel confident about it.”