The Panathinaikos left-back came off with an ankle injury in the 25th minute of Croatia's 1-0 win over Australia in Bahia on Friday in their last friendly before the showpiece event.
Pranjic twisted his ankle under a challenge, with Kovac unimpressed by Australia's physical approach to the clash.
Asker if the 32-year-old would be available for the tournament opener on Thursday, Kovac said: "I still don't know.
"His ankle is swollen. We'll see. I hope he'll be ready.
"Sime Vrsaljko played well on that side. I have an idea of how we'll play against Brazil."
Star striker Mario Mandzukic, suspended for the opener against Brazil, is OK after receiving a couple of knocks late in the win, according to Kovac.
Kovac said it was far from the friendly encounter he expected, but was particularly pleased with his team's second half as Nikica Jelavic scored a fortunate 58th-minute winner.
"I didn't think this was a friendly encounter, but in the second half we knew how to respond," he said.
"In the first half we were static, but in the second we were much better and with that part I'm pleased.
"We will be ready for Brazil and I hope we'll play like the second half against Australia."
