NASHVILLE, Tenn.—Coach Mike Vrabel heard that his banged-up Tennessee Titans were being written off after the left side of their offensive line was ruled out a day before hosting the streaking San Francisco 49ers. Then they lost their backup left tackle to a positive COVID-19 test Thursday morning, forcing them to use their 52nd different starter and 88th player this season. “We’re not dead yet,” Vrabel said. Alive and kicking. Randy Bullock booted a 44-yard field goal with 4 seconds remaining and the Titans rallied from a 10-point halftime deficit to beat the 49ers 20–17 Thursday night. The Titans (10–5) are a win or a Colts’ loss away from clinching their second straight AFC South title despite losing three of their past four games to drop from the AFC’s top seed to third. Indianapolis plays Arizona on Saturday. Tennesse moved to second in the AFC. Ryan Tannehill threw for just …