Shimano BB's are not compatible with SRAM/Truvativ cranksets.

However, that plastic piece is not exactly an essential part of the crankset/BB interface as long as it's dry. You can do a race on it while you wait for a new one to come in. It doesn't provide any structural support; just acts as a seal connecting both cups to each other to keep out water.

The seals that the crankset arms press into when tightened down should provide all the water tightness necessary, in theory.