I've got a realistic trade that could fill the backup SF spot and potentially the stopper role: Gee and a 2nd round pick for Young.
With Gay, Mayo, Henry and Allen, Young is expendable. Allen in particular makes him expendable, because he's of the same ilk and will be taking on Young's role.
Plus, Hollins supposedly wants another PG (Conley is their only true PG; Mayo/Vasquez are set to handle the backup duties), but Heisley is supposedly too cheap to okay that. They could waive Gee, who's contract is not guaranteed, then take the open roster spot/savings from that to sign a backup PG.
For the Spurs, this would put them at least two deep at every position, give them another true SF and a potential answer to the stopper role. It would also allow them to continue to get younger and more athletic (granted, Gee is younger and more athletic than Young, but whereas I doubt Gee will be in the rotation, Young more than likely would be), while barely increasing payroll.