Complex sentence: In 2017, Romine began the season as the backup to Gary Sanchez but after an injury to Gary Sanchez on April 8,2017, Romine became the Yankees starting catcher.
Split into simpler sentences: In 2017, Romine began the season as the backup to Gary Sanchez. After an injury to Gary Sanchez on April 8,2017, Romine became the Yankees starting catcher.