He has already to an extent. But like Bojan said, he's in Xavi's mould, he can turn it up for the most important moments even if he will look a bit jagged and sluggish in other games of the season. He's definitely less durable and cannot do 60 minutes of his top performance game-in game-out. To compare it, he definitely doesn't have the same influence Pogba has on United's offense anymore. If you take Luka out Isco, Kroos and Asensio usually pick up the slack. But it's one of the reasons why Kovacić was missed more than people probably felt it this season over these 2-3 months. Vastly different styles, Kova's runs and movement are more like prime-Kaka, but influence and collectedness is very much similar from both.
Basically he's declining, but very slowly.