It's probably not a case of him being deemed surplus that early. The fact he was brought along for the pre-season tour is a sign that getting rid of him wasn't always in the cards, but now with the Neto signing and the club finding themselves in a position of being almost forced to sign Felix in order to push through the Gallagher sale, the circumstances with Sterling have only recently changed. This one has Jorge Mendes' grubby little hands all over it, with him being the agent of both Neto and Felix.
All in all it's no doubt very chaotic and I'm not at all convinced Sterling is actually moveable given his huge salary. I think at this point the best we can hope to get for Sterling is a loan deal.
The nuclear option would be to simply cut his contract by mutual termination and allowing him a free transfer, if he were to accept that without demanding the club pay him off. That way his large salary could then be more affordable for his next club because they wouldn't have to pay a fee to sign him. But this idea is dependent on Sterling negotiating an acceptable contract with another club first, which is not a certainty even with free agency. This would of course result in Chelsea having to account for his full remaining amortisation value as a big loss on the books for this season (£28.5M) but then again getting his massive salary out would balance it out to roughly a net zero compared to keeping him for the season (£26M a year for amortisation + salary). Basically Sterling's cost on the books would stay roughly the same for the current season, give or take a few million, but then save around £26M/y in costs for the remaining two years. If he has absolutely no future at the club, even this option is better than keeping him around stinking up the place and being unhappy.