49ers rule out two defensive starters for Sunday’s Week 15 game vs. Titans

The 49ers on Friday announced two defensive starters will not be in uniform to face the Tennessee Titans this weekend.Linebacker Tatum Bethune and defensive end Sam Okuayinonu were ruled out of Sunday’s game due to high-ankle sprains.The 49ers also listed backup linebacker and special-teams contributor Nick Martin as out as he remains in the NFL’s concussion protocol.Bethune, who is tied for the second-most tackles on the 49ers, will sit out his third game in a row. Bethune has 64 tackles on the season.Okuayinonu will be out for his second game. He started nine of the 49ers’ first 12 games and has two sacks on the season.The 49ers opened the practice windows this week for defensive lineman Yetur Gross-Matos and quarterback Kurtis Rourke. Neither will be activated for the game, coach Kyle Shanahan said.Linebacker Curtis Robinson is expected to make his third start in place of Bethune on Sunday at Levi’s Stadium.Veteran linebacker Eric Kendricks is not an option to be elevated from the practice squad for Sunday’s game.Kendricks, who joined the 49ers on Nov. 26, sustained a grade 1 calf strain and was not available to practice this week.If Kendricks were...