Ideally I aim to spread the manure before growing onions and garlic, in October for garlic and before Christmas for onions – so that it is mostly taken in by worms before planting autumn salad, after the harvest of allium bulbs. This helps to keep slug numbers down in those tender leaves. Through August I am planting salads into soil that has only a thin residual mulch of last winter’s compost and manure.
However, in a dry year such as this, you should be alright with compost or well rotted manure that has been spread this summer, and I would expect your autumn salads to grow well.