The Viola have now conceded 14 goals in the last seven games.
"I have to compliment our opponents, even if we should be more ruthless because we can't always score four goals to get a victory," Montella told Sky Sport Italia.
"We had so many opportunities to extend our advantage, but in defence we've got to maintain concentration.
"Borja Valero got a brace? If he starts scoring from outside the box as well, then it's clear this is the complete player."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball