Herndon, eligible to play after a four-game suspension, is listed as week-to-week with a hamstring injury and is not expected to play Sunday against the Dallas Cowboys.
"It's legit, it's a legit hamstring," coach Adam Gase said Wednesday. "It's not ideal."
Typical of the Jets' recent luck with injuries, Herndon pulled his hamstring before he was allowed to return to practice. It happened last Friday when he was running routes on his own.
"I feel bad for him because he's been waiting to get back," Gase said. "... I know the guys were looking forward to getting him back, too."
The Jets' moribund offense, which has produced only two touchdowns in four games, is desperate for a spark. Darnold, returning from a four-week bout with mononucleosis, will certainly help matters. But they also were expecting Herndon to generate production from the tight-end position.
The Jets' tight ends have a league-low five receptions, according to ESPN Stats & Information. Asked his initial reaction upon learning of Herndon's injury, Gase smiled and said, "I don't know if I can say."
Herndon, a fourth-round pick in 2018, emerged last season as one of the team's most promising young players. He developed a rapport with Darnold and finished with 39 catches for 504 yards and four touchdowns. Gase, impressed with Herndon's skill set, referred to him as a "unicorn" in the offseason.
Herndon was suspended for violating the league's substance-abuse policy, stemming from a June 2018 arrest for drunken driving. He pleaded guilty in January.
By rule, Herndon was permitted to be at the team's facility during the suspension, but he wasn't allowed to practice with the team.
"It's tough to go through what he had to go through because you're around and can't do anything," Gase said. "To finally be able to get a chance to be on the field, and now he can't."
In other injury news, middle linebacker C.J. Mosley is expected to miss a fourth straight game with a groin injury. Gase said the injury, suffered in Week 1, was "a little more severe than any of us felt."
The team believes Mosley is close to returning, perhaps next week against the New England Patriots.