Croatia's midfield icon Luka Modric suffered a muscle injury in his side's 2-2 draw with Czech Republic Friday that coach Ante Cacic said may have caused his side's loss of a two-goal lead.

The Real Madrid star came off in the 62nd minute of the game with what Cacic called "a muscular problem".

Cacic said Modric was his side's "most important player" and his absence along with flares thrown on the pitch by Croatian fans could have caused a meltdown.

Modric is now a major doubt for Croatia's final Group D game against European champions Spain on Tuesday.

Croatia had been cruising at 2-0 up, a score which could have seen them through to the last 16, but Milan Skoda pulled a goal back, and after the flare drama, Tomas Necid equalised from the penalty spot in injury time.

"That was the main problem. Our concentration dropped when he (Modric) left the field," said Cacic.

Croatia face more problems as UEFA has said it will open an investigation into the pitch incidents.