FLORHAM PARK, N.J. — Chan Gailey doesn’t like the word mystery, not when it comes to Christian Hackenberg. But there may be a few other descriptors he’ll allow.

Teachable. Improving. And hey, even talented.

“I think he’s a developmental player at this point,” the Jets’ offensive coordinator said Thursday. “There’s a lot of talent there. The sky’s the limit, but we don’t know yet.”

That last part might, in fact, put Hackenberg in the mysterious category, but despite no playing time, Gailey said he’s seen clear signs of maturation. And while the Jets don’t seem to know if he has a future as an NFL starter — his preseason appearances were raw, to say the least — he has impressed his teammates, Brandon Marshall said.

“I look at guys, especially at the quarterback position, as far as their approach — what they do in the classroom, how do they lead, do the guys follow. He’s been doing a great job there,” said Marshall, who acknowledged that it’s difficult to get a bead on a guy playing on the practice squad. “He has all the tangibles.”

Marshall appreciates that Hackenberg has a mastery of a pro-style offense; it’s something he did under Bill O’Brien at Penn State. But pro-style isn’t the same thing as NFL-ready, and Gailey knows it.

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“He’s learning every day about when to throw the ball, how to throw the ball up here,” Gailey said. “It’s different up here than it is in college. He’s learning a great deal . . . He understands the game better, he understands the defenses better, he understands protections better and that’s all part of the growing process.”

Jet streams

C Nick Mangold (ankle) returned to practice, but WR/KR Jalin Marshall did not practice because of a concussion he suffered against the Patriots that he reported Wednesday, coach Todd Bowles said. Brandon Marshall (knee/foot) also did not practice, along with S Calvin Pryor (concussion), RT Breno Giacomini (back/shoulder), DT Steve McLendon (hamstring) and LB Lorenzo Mauldin (ankle).