LOS ANGELES -- The Chargers ruled out wide receiver Keenan Allen on Wednesday for Thursday night's game against the Las Vegas Raiders because of a heel injury.

Allen's injury comes at an inopportune time for the Chargers, who head into this game with backup quarterback Easton Stick making his first NFL start. The Chargers' starting quarterback, Justin Herbert, is out for the season with a broken right index finger.

"He provides that middle of the field target, or, 'Hey, if he's playing outside, I can trust it,'" coach Brandon Staley said Monday of the importance of Stick having Allen.

Allen is having his best season. Last week, he set the Chargers single-season record for catches with 108. He leads the NFL in receptions and is fourth in yards.

Helping the Chargers will be the return of receiver Joshua Palmer, who was on injured reserve with a knee sprain since Week 9. Palmer had five catches for 133 yards in Week 7 against the Kansas City Chiefs before suffering the injury in Week 8.

Raiders interim head coach Antonio Pierce has been noncommittal about which quarterback would start Thursday night, but a source told ESPN's Adam Schefter that rookie Aidan O'Connell is expected to remain the starter against the Chargers.