By Alek Arend This Thursday night, the Los Angeles Chargers face a pivotal AFC West test against Patrick Mahomes and the Kansas City Chiefs. Unfortunately, Justin Herbert won’t have his full passing attack available. Chargers wide receiver Keenan Allen is “unlikely” to play this Thursday night. Allen exited the team’s season opener vs. the Raiders with a hamstring injury. Los Angeles was still able to come away with the 24-19 victory. The good news, meanwhile, is that Allen’s hamstring injury isn’t believed to be serious. He should be able to return for the team’s third game of the regular sea…