Complex sentence: He struggled in the American League the rest of the season, with a 4.99 ERA in 45 games; 1988 was better, posting a 3.51 ERA with 18 saves.
Split into simpler sentences: He struggled in the American League the rest of the season, with a 4.99 ERA in 45 games. He improved in 1988, posting a 3.51 ERA with 18 saves.