The Chargers’ Rashawn Slater avoided serious injury during practice Tuesday. The memory of last year’s ruptured left patellar tendon injury, and an entire season spent navigating life without him, remains fresh. There are countless variables that will determine the Chargers’ fate this season, from the obvious to the subtle. But one reality stands above the rest: They need their left tackle on the field. So, yes, there was a sense of panic Tuesday when Slater left practice.