Gase said Herndon was running routes last Friday when he got hurt. He added the injury is "legit" and could force Herndon to miss Sunday's Week 6 game against the Dallas Cowboys at MetLife Stadium.
Herndon has yet to make his 2019 debut, having served a four-game suspension for a violation of the NFL's substance-abuse policy. In January, he pleaded guilty to a charge of driving while intoxicated after being involved in a car crash in June 2018.
A fourth-round pick in the 2018 draft, Herndon was a pleasant surprise as a rookie. He caught 39 passes for 502 yards and four touchdowns and finished 18th in DYAR (defense-adjusted yards above replacement) among tight ends with at least 25 targets, per Football Outsiders.
The impact of Herndon's absence is evident in the production of his replacement, Ryan Griffin. Through five weeks, Griffin has five receptions for 17 yards. While some of that can be attributed to quarterback Sam Darnold's absence due to mononucleosis, Griffin didn't eclipse 442 receiving yards in any of his first six years.
Darnold received medical clearance to return to the field in Week 6, but Herndon's hamstring injury will likely mean the Jets offense still isn't operating at its full capacity against Dallas.