Keep a close eye on those lesions.
They could be insect damage, in which case it's merely cosmetic, but they could also bacterial leaf spot. If those spots turn brown, expand etc and/or you notice tiny black spots on the leafs it's leaf spot.
Copper-based fungicides such as Boredeaux mixture usually work but many lettuce cultivars find copper toxic. The best solution is to destroy the affected plants, rotate cultures and switch to a leaf-spot resistant cultivar.
