The New York Jets lost another key offensive weapon ahead of Week 2.
"We'll have to make some adjustments," Gase said.
The coach described the veteran wideout as "probably our best player on offense the last two years," according to Kim Jones of NFL Network.
Crowder has been battling a hamstring injury this week. He practiced on a limited basis Wednesday and missed Thursday's session, according to team reporter Ethan Greenberg.
The 27-year-old, who led the Jets in receiving last year, racked up seven receptions for 115 yards and one touchdown during a 27-17 loss to the Buffalo Bills in the season opener.
Crowder's absence is another hit to the Jets' offense, which lost star running back Le'Veon Bell to injury this week and has yet to see second-round rookie wide receiver Denzel Mims on the field. Bell and Mims were placed on injured reserve and will miss at least the next three weeks.