Combine rhubarb, sugar, and water; cover and cook just till tender, about 5 minutes.
Drain thoroughly, reserving syrup.
Drain pineapple, reserving syrup also.
Combine the pineapple and rhubarb syrups and add water to make 1 3/4 cups.
Heat to boiling; add to gelatin and stir to dissolve.
Add lemon juice.
Cool, then chill until partially set.
Fold in rhubarb and pineapple; pour into 1-quart mold.
Chill until set.
Unmold on salad greens.