The Lakers opened up a 15th roster spot Friday, trading Rajon Rondo to the Cleveland Cavaliers in exchange for Denzel Valentine, who has a partially guaranteed contract and is expected to be waived. Johnson is now in competition with veteran point guard Darren Collison, who’s also on a 10-day hardship exemption contract, for the final roster spot. “I know if I do my job to the best of my ability, I think I can make it hard for (Lakers vice president of basketball operations and general manager Rob Pelinka),” Johnson said. “So that’s what I’m focused on: doing my job to the best of my ability and winning some of these games here.”