Cameroon international Song, who is currently out with an ankle injury, made 31 appearances for the Hammers during a loan spell last season.
"We just hope the medical shows he's in a sufficient state for us to gamble," Sullivan said.
"He couldn't physically pass a medical tomorrow, but if the medics say it's just a technical thing and he's only a couple of weeks away you'd take that gamble and hope they're right."
Sullivan also said that signing Song on a three-year deal would be a "huge commitment" for the club, adding: "He's a big earner."
West Ham kicked off the new league season with an impressive 2-0 win at Arsenal, where Song spent seven years before joining Barcelona. The Hammers, beaten 2-1 by Leicester on Saturday, host Premier League newcomers Bournemouth at the weekend.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by Peter Rutherford)
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