The Australia internationals are both in the final year of their deals after impressive debut seasons in Paisley.
While chairman John Needham said this week St Mirren were speaking to the players' agents about new contracts, the prospect of keeping them beyond this season appears unlikely.
Robinson said: "That was a verbal conversation that myself and [vice-chairman] Jim Gillespie have had with the boys and their representatives and as yet both are keeping their options open, which they are more than entitled to do.
"We will continue to try but also continue to look for other people. We have to be a step ahead of other people so that's what we are trying to do.
"We want both boys to sign, we have made that very, very clear and we will still chase that but it's up to me and the club to be prepared if they don't sign and we have to sign replacements. It's still very early."
St Mirren rebuffed interest in midfielder Baccus from Aberdeen last month and while they may face a big decision over whether to cash in during the January window, Robinson says the club won’t sell players “for buttons”.
"There's a balance," he said. “We had an offer in the window which we just didn't think met the value we thought the player was worth. Fair play to the board, they stuck by that.
"We also have to weigh up whether keeping players would get us an extra two or three positions in the league. You get extra prize money for that as well. Certainly it will be a calculated decision.”