The Croatia international winger is being tracked by Bayern and Tottenham while he's at the World Cup.
Kovac worked with Rebic when coach of Eintracht Frankfurt and admits he remains a big fan.
"He's an excellent player, I'm glad he plays so well in the World Cup. He would be an asset for all teams," said Kovac.
The 24 year-old scored nine goals in 28 appearances last term, including a brace in Frankfurt's shock German Cup final victory over Bayern Munich.