Complex sentence: The remainder of his tenure at Valencia would prove disappointing: the team would slump to 15th in the league, only two points above the relegation zone, as well as finishing bottom of their Champions League group.
Split into simpler sentences: The remainder of his tenure at Valencia would prove disappointing. The team would slump to 15th in the league, only two points above the relegation zone, as well as finishing bottom of their Champions League group.