The Croatia international revealed during the World Cup that he would seek a move away from the Bernabeu this summer as more than half of his appearances over the past three seasons have come from the bench.
Chelsea have been strongly linked with Kovacic over the past few days, with an initial loan move being touted, and Lopetegui concedes that he may be forced to delve back into the transfer market if he loses the central midfielder.
"There's nothing 100% certain," he told reporters. "Mateo has said he wants to leave, I've said he is an important player. If he wants to leave, we'll look to find solutions.
"Obviously if Mateo does leave, we'd look for players of his level. We need to have a balanced team and he's an important player."