Complex sentence: Tagovailoa was backup to Alabama quarterback Jalen Hurts throughout the 2017 season, but replaced Hurts in the second half of the 2018 College Football Playoff National Championship due to ineffective play by Hurts.
Split into simpler sentences: Tagovailoa was backup to Alabama quarterback Jalen Hurts throughout the 2017 season. He replaced Hurts in the second half of the 2018 College Football Playoff National Championship due to ineffective play by Hurts.