He was promptly hauled off at half-time as the Blues lost 3-1 to the Dutch giants, managed by Martin Jol after he finally turned down Fulham this week.
Cech has been ruled out for at least a month having torn his right calf muscle in training.
But boss Ancelotti insisted he always planned to play Henrique Hilario after the interval.
He said: "We just have to hope Cech recovers quickly. But we have two other goalkeepers and have to keep faith with them.
"Everyone could see Turnbull didn't have a good performance but that can happen to any young keeper and I still trust him."