Here’s another way (usually easier, because you do not have to be exact while selecting):
hover with your mouse pointer over the new mesh (over a Vertex, when in Vertex Select Mode, and so on..)and press L. This selects everything that is connected to that Vertex( or Edge, or Face…).
Or select one Vertex, Edge or Face (or just any part) of that new Mesh and then pres CTRL+L.