#1 Too big a job to handle in the long run

Many people would call these reasons being too short-sighted, coming on the back of one bad result. Yes, it was one bad night in Roma. Results sometimes do not go in one's favour, agreed. But the manner in which Valverde's Barcelona lost the plot against AS Roma looked extremely worrying.
He looked clueless and passive standing on the touchline at the Stadio Olimpico. As discussed earlier, Valverde was never a manager who ever went gung-ho about tactics. He has a rigid plan which he adheres to.
This plan has yielded them success in La Liga and at other times, the brilliance of Lionel Messi has come to the rescue. But when the going got tough against AS Roma, Valverde failed to try and even change the narrative of the match. He failed to take any risks, take any big step that could have stopped the gigantic upset in Rome.
Also, the approach with which he deployed his team from the start reeked of a defeatist attitude, the one which does not fit with the club's history and philosophy. His playing style, his late substitutions, his choice of incoming players at the club have all been questioned in recent times.
Valverde failed the biggest test in his Barcelona stint in a competition which is considered as the holy grail in club football. Yes, he has done a splendid job in stabilizing the club but his tactical naivety, his risk-free approach and his failures in the Champions League does not warrant him to stay at the club beyond this season.