Luke Walton will be sidelined indefinitely after suffering a partially torn ligament in his left ankle in Game 4 on Saturday night, the Lakers announced today. Walton underwent an MRI exam this morning, which revealed the extent of the injury. Walton’s condition will be re-evaluated in about one week.

Meanwhile, Lakers coach Phil Jackson told our Ramona Shelburne that Trevor Ariza will play tonight in Game 5 despite suffering from a sprained right ankle. Either shooting guard Kobe Bryant or power forward Lamar Odom will serve as Ariza’s backup at small forward, Jackson said.

“I think Trevor will be able to go tonight, but Luke is out indefinitely and we’ll have to wait and see what’s going to happen with that,” Jackson said this morning. “I think (Walton) was instrumental in helping us win that Game 4. … Trevor didn’t complain about it at all. But when I watched him shoot a little bit, I was seeing if he would favor it or not. He caught me looking at him, smiled and said, ‘I’ll be all right.’ So I think he’s going to be fine.”