Complex sentence: He began his career at A.C. Milan, making his Serie A debut at age 17, and in 2009 joined Padova, whom he helped avoid relegation from Serie B in his only season there.
Split into simpler sentences: He began his career at Milan, making his Serie A debut at age 17. In 2009, he joined Padova, whom he helped avoid relegation from Serie B in his only season there.