Monaco must overturn a two-goal deficit against Valencia if they are to qualify for the UEFA Champions League group stage.
Coach Leonardo Jardim described Monaco's 3-1 defeat at Valencia as unfair, lamenting missed opportunities and poor defending away from home.
Valencia hold the upper hand in the UEFA Champions League play-off tie thanks to Sofiane Feghouli's strike four minutes from time at the Mestalla on Wednesday.
Earlier in the second half, Daniel Parejo had restored Valencia's lead after Mario Pasalic's 49th-minute equaliser cancelled out Rodrigo de Paul's opener in the Spanish city.
It was not the scoreline Monaco had wished for, with Jardim rueing a brilliant finger-tip save from debutant Valencia keeper Mathew Ryan, who pushed Bernardo Silva's strike onto the post in the first half, but the Portuguese boss has not given up hope of reaching the group stage.
"The result is unfair for two reasons. First, we hit the post. If that had gone in, the goal would have changed the rest of the game," Jardim was quoted as saying by UEFA.com.
"Also, when we were playing better Valencia scored their second and third goals.
"Conceding goals like today, from balls across the area, is bad. This is not normal for our team and we need to correct it.
"We have to score two goals at home, we have the ambition to try and win this tie."
Monaco welcome Valencia to the Stade Louis II for Tuesday's return leg.comments