The 30-year old, selected ahead of Donovan Ricketts, produced some vital saves in the 1-0 Western Conference semi-final first leg win over New York Red Bulls and Arena was delighted with how he performed on such a big occasion.
"He had an outstanding game," Arena said. "Obviously some key saves, several of them in the second half. He was outstanding."
Saunders' Galaxy teammates Landon Donovan and Mike Magee were also clearly impressed by his showing.
"Josh was real aggressive coming out," said Donovan. "That relieved a lot of pressure for us."
Goalscorer Magee added: I feel like Josh is a beast every game, but today he was huge. If they get one of those goals - and nine out of 10 times, they're going to score those - it changes the whole game. And those were pretty much sure goals that he makes freakishly good saves on, so I'm pretty happy that he's back there.
"Any time you have a guy like Donovan Ricketts on the bench your starter's got to be pretty damn good. And that's obviously the case."