Was green manure a good idea??

I planted up a spare bed with the green manure, Phacelia a couple of months ago.  I've cut it down and left it wilting on the surface.  The idea was to dig it in but I am suddenly very short of time and the bed also has alot of bindweed in it.  So my question is, if I cover the lot with black plastic till autumn and then dig it in, will the wilted green manure have lost all of it beneficial nutrients and was it all just a waste of time.  Thank you for reading

best to cover with plastic as the bind weed is really difficult to get rid of by digging.  Ant tiny bit of root that is left will grow again.  I have been battling with the stuff in our flower bed for yrs.