To regain the leaf area, you may use foliar fertilizers preferably natural based. If you are interested, i shall send you a combination product which is of herbal based with sea weed extracts, amino acids, and biostimulants. Use few granules of ammonium humate also as a basal application
You may spray Urea. But look at the stress level of the crop. Thats why i recommended the natural based nutrients. You may go for urea once the crop recovered fully. Humic acids and sea weed based products take care of crop stress.