Eden Hazard is set to miss Real Madrid's crucial clashes against Manchester City and Barcelona after suffering a hairline fracture to his right ankle.
The Belgium forward hobbled off in the second half of the 1-0 loss to Levante on Sunday (AEDT) and could be seen on the bench receiving treatment on his lower right leg.
Hazard only recently returned from a fractured right ankle so the sight of him appearing to have another problem in the same area was alarming for Zidane.
Madrid faces Pep Guardiola's City in the first leg of a UEFA Champions League Round of 16 tie on Thursday (AEDT) before a Clasico date with new LaLiga leader Barcelona next Monday (AEDT).
"It doesn't look good," Zidane said.
"It can become weak where you've had an injury. It is a blow, let's see.
"It's sore now but we'll see tomorrow when we do more tests."
It was a thoroughly miserable outing for Madrid as they suffered a surprise defeat thanks to Jose Luis Morales' stunning late effort.