The 38-year-old recalled his time spent with Mario Mandzukic at Dinamo Zagreb and admits he is surprised at how the striker has developed at Bayern Munich
The Croatia international has netted 15 times for Bayern in the Bundesliga this season and Kovac, a former team-mate of Mandzukic, concedes the forward has exceeded his personal expectations.
Speaking to TZ Online, the veteran centre-back said: "Before the season started I had classed him as a reserve player, but his hard work is paying off and he has taken the league by storm.
"I'm very familiar with Mario. We played together at Dinamo Zagreb and he has really made a huge step forward and changed very much in contrast to his early career."
Ahead of Wednesday's DFB Pokal clash between Borussia Dortmund and Bayern Munich, the 38-year-old believes the latter will be keen to avenge the heartbreak of losing to Jurgen Klopp's side in the final of last year's competition, labelling the defeat as a "fluke".
"Bayern want to show that these results, like the cup final defeat to Dortmund in Berlin, were just flukes," he said.
"For Dortmund this year, the cup is probably their only realistic title. The pressure of the situation is finely balanced.
"These teams know each other very well. They both want to win and need to win. What a game!"