It's a possibility. The US already has special ops and military advisors in Syria, as do we. Their attention until now has been on ISIS, particularly in northern Syria by providing airstrikes against ISIS and supplying some (not enough) weaponry to the Kurdish militias. I actually have no problem with any country supplying the Kurds in Syria because they have no allies and are taking the fight directly to ISIS but if Assad falls now, ISIS and al-Nusra will focus their full attention on the Kurds, Shias, Sunnis and Assyrian Christians in the north because they won't have to worry about Assad anymore.
It's happening as we speak. Like I said, Islamic State have taken advantage of the US airstrikes on Assad's base to launch an offensive. Jihadi groups and al-Qaeda affiliates are doing the same. The US just provided air support for a terrorist counter-offensive. The chemical attacks are disgusting but tonight wasn't the time to start launching more than 50 tomahawk missiles at the regime.