His 8-10 week recovery brings him back in late November or early December. The Lightning will need to hold on until then to stay in playoff contention. Long-term back injuries can be tricky to keep from re-injuring. For the success of the Lightning the organization better hope this doesn't turn into a chronic issue.
With Vasilevskiy out the Lightning will likely turn to Jonas Johansson and Hugo Alnefelt for the time being. Expect the Lightning to go searching for a better option through trade, waivers or free agency in the coming days though.