Complex sentence: The New York Mets signed Pérez as a free agent in 2000 and was a spark plug for them in the 2000 NLCS, before being traded to the Chicago White Sox four years later.
Split into simpler sentences: The New York Mets signed Pérez as a free agent in 2000 and was a spark plug for them in the 2000 NLCS. He was traded to the Chicago White Sox four years later.