A simplified theory for the crystallisation of biodegradable polyesters induced by polymer chain scissions during biodegradation is presented following a theory developed by Han and Pan. The original theory is greatly simplified so that it becomes very straightforward to use and the number of material parameters is significantly reduced. Furthermore it is demonstrated that the spherulite structure widely observed in polymers can be taken into account using the theory. The simplified theory is fitted to the experimental data of poly-l-lactic acids (PLLAs) obtained from literature. It is shown that the simplified theory is equally able to fit the data as the original one. It is also shown that the theory can fit degradation data for PLLAs of different initial degrees of crystallinity with spherulite structures.