If Ainge is confident that the O'Neals are healthy for the playoffs, or at least one of them, I doubt he'd hesitate in trading Erden, regardless of where Murphy is going. Erden is an end of bench player, the only way he sees playoff minutes is if both O'Neals are out - and even in that case it'd only be situational minutes, when someone gets in foul trouble and stuff like that. No way he makes the regular playoff rotation.
In that scenario where Boston has guaranties they'll land Troy Murphy, he'd probably shop Glen Davis, not Erden.