Fresh from winning the Champions League with Real Madrid, Modric has displayed sumptuous form in France, volleying a marvellous winner to crown a man-of-the-match display in the opening 1-0 win over Turkey.
The 30-year-old was similarly impressive as Croatia cruised into a 2-0 lead versus Czech Republic, only for his departure with muscle tightness to herald a collapse to 2-2 against a backdrop of crowd disturbances.
Victory over the double defending champions would still see Cacic's men progress as group winners and he was pleased to report a scan showed Modric had suffered no serious damage.
"Talking about Luka Modric, he did a MRI scan. There is no extension, there is no rupture of the muscle but I will decide tomorrow whether he will pay or not," Cacic told a pre-match news conference in Bordeaux.
"He is a really important player for our team. It will be a special moment if he can play."
Another Croatia star familiar to Spain fans is Modric's midfield accomplice - Ivan Rakitic of Clasico rivals Barcelona
"Talking about Ivan Rakitic, I can say I've been the coach of this team since October last year and I think he is playing some of the best games in this national team," Cacic added.
Rakitic got on the scoresheet after Ivan Perisic opened the scoring last time out.
Winger Perisic believes the dedication and endurance of captain Darijo Srna, who returned home for his father's funeral before starting the Czech game, will continue to serve as an inspiration to the team.
"Darijo, he's playing for the Croatian team for 15 years," Perisic said. "He is a great person. I can only say positive things about him
"He went through a really heavy moment - not many players will come back the day after his after their father's funeral for the team
"He is so important for this team, he is our captain. This can only motivate us even more and we want to go as far as possible because of him."