Modric left Spurs in 2012 before Kane had found his form.
Kane, 25, has been criticised from many sections of the media this season for showing tiredness in his performances, after a gruelling World Cup that saw him lift the Golden Boot with six goals for England.
He has scored the same amount this season for Tottenham in 10 league and cup appearances, including most recently a brace against Huddersfield. But the vigour that saw Kane shine last season, with 41 goals, is somewhat lacking.
Real Madrid midfielder Modric discussed former teammate Kane ahead of England's first meeting with Croatia since the semi-finals of the World Cup, stating the former Leyton Orient loanee can get even better.
"He's scoring a lot in recent years, playing great and practically putting Tottenham in the title race, which is a big thing considering other clubs are spending a lot of money," Modric said.
"He's already one of the best forwards in the world. If he continues to develop on the same path he can be even better."
Kane is expected to captain England on Friday night, as the Three Lions do battle with Croatia at the HNK Rijeka Stadium behind closed doors. Croatia were punished after a swastika was marked on their pitch before a Euro 2016 home qualifier against Italy.
In what is a UEFA Nations League clash, neither side will be looking to lose the match after suffering defeats to Luis Enrique's Spain last month. England and Kane then play Spain on Monday night.