The Sweden international has yet to agree terms on a new deal at The Hawthorns and will become a free agent next summer if no contract extension is put in place.
Foster said: "The chairman needs to lock him in a room and hopefully get something sorted out.
"If you take everything into account I'd much rather be playing with him and Gareth McAuley than the majority of defensive pairings.
"It's people like Jonas we need to be keeping. He is such an important part of our team."