SAN FRANCISCO: Chris Borland, a 24-year-old linebacker for the San Francisco 49ers, told ESPN on Monday (Tuesday in Manila) he is retiring from the National Football League over health concerns due to repeated head injuries. He becomes the most prominent NFL player to turn his back on a rich contract, a four-year deal worth just under $3 million in Borland’s case, over concerns about brain injuries. The 49ers lost linebacker Patrick Willis last week. He announced his retirement at age 30 after playing only six games last season due to a toe injury. Borland said the decision by Willis played no role in his decision, which includes plans to return to college and study sports management.