Complex sentence: Professional baseball teams generally keep three to four middle relievers who are used to replace starters who either pitch ineffectively or are unable to start the game or continue pitching during the early innings for any reason (e.g. injury).
Split into simpler sentences: Professional baseball teams generally keep three to four middle relievers. They replace starters who either pitch ineffectively or are unable to start the game or continue pitching during the early innings for any reason (e.g. injury).