I'm sure Roma will sell him to us in January regardless of how they fare in Europe. After all, they managed just fine without him after his injury and by the time he's 100% fit to play, Roma's fate in the group stages probably will already be decided.
Their chiefs are not stupid, they know it's a matter of time, especially with some reliable media outlets reporting that he's very keen on the move. They're already planning for life without him, it's quite clear. The good fee they'll receive for him will allow them to sign a quality replacement or just sign Rabiot on a free next summer.