The midfielder suffered a painful knock in the game against die Fohlen and the new signing had to be rushed to hospital with his vision temporarily impaired
The Spaniard was caught by the Austria international's hand as the two challenged for the ball in the air after a corner, and the Bavarians quickly withdrew the player after assessing the damage sustained.
Heynckes refused to reveal the extend of the the injury, but admitted to reporters that the player "can't see with his left eye".
The midfielder was substituted right before the end of a first half in which his side had fallen behind to a penalty from Thorben Marx, only to ultimately salvage a draw courtesy of Xherdan Shaqiri.
It is not yet clear whether Martinez will recover in time to feature in the DFB Pokal clash with Augsburg on Tuesday night.