The Austrian defender only started three games this season, the last three, and did enough to make Neil Warnock think about keeping him on.
It was thought Ertl would be a priority to get rid of this summer.
"Ertl has done alright in the final three games," Warnock told the Croydon Advertiser.
"He is desperate to stay at the club, he has a tremendous work ethic and I've been happy with him over the last few weeks.
"It's not absolutely urgent that we sell him in the summer - we are just looking at ways we can save money."