Hi, I don’t think you need to worry about it burning the roots of your lettuce and spinach. Phosphorous promotes strong root growth and is one of the things to throw in the hole when you put in a new shrub, for ex.
Lettuce and spinach have shallow roots, so it’s no harm to leave it out. It might help a little, though, esp. the other nutrients like calcium and magnesium. If you do rake in some bone meal before you sow the greens, it probably will still be working (dissolves slowly) when you harvest them and put in something new.
Someone else here might have a more scientific explanation on which plants benefit most from phosphorus, and when.