Complex sentence: Durkan started training the horse but was diagnosed with leukemia and suggested that Aidan O'Brien train Istabraq while he was ill. The agreement was that when Durkan recovered, he would take over the training.
Split into simpler sentences: Durkan started training the horse but was diagnosed with leukemia. He suggested that Aidan O'Brien train Istabraq while he was ill. The agreement was that when Durkan recovered, he would take over the training.