Foster has also urged Gera to take heart from his own recovery from knee setbacks.
The keeper said: "It's a big blow for Zoltan. I could see from where it was that it was going to be a bad one.
"He seemed to twist badly and I feared the worst straight away and, sadly, it looks like it is.
"I know what he is going through having suffered a cruciate ligament injury twice myself - at Stoke and then Manchester United.
"It's horrible but both times I've come back strongly and Zoltan will come back from this and be a better player for it. He's got the right attitude and mentality."