The correct verb form for 'I played' in the sentence is 'jugué.'
In the given sentence, the correct form of the verb 'jugar' to match the subject 'yo' (I) is 'jugué,' making the complete sentence 'Tú jugaste muy bien en el partido el sábado, pero yo jugué fatal.' In this context, 'jugaste' refers to 'you played' while 'jugué' refers to 'I played.'