Treviso's last win came against Cardiff Blues back in February, and despite dominating proceedings early on, Casellato believes their own mistakes cost his side once again.
But with the season still in its infancy, the 46-year-old insists there is plenty of time of turn things around.
"It is clear that we need to improve on Friday at the home of Connacht," he told La Tribuna di Treviso.
"We will have to have a lot of discipline as there is always wind there, and it will be a battle in the line-out and scrum.
"But we have the heart to win, although obviously that has not been enough recently as we have been making too many mistakes.
"It is of course frustrating to see teams ahead of us that are higher in the standings, such as Newport Gwent Dragons who beat us with a driving maul but the rest of the game was on our level.
"Edinburgh too only just beat us here, and Zebre have exploited the double round at home.
"We'll have a few months to get ourselves back up the table. I continue to have confidence, as bad luck cannot last all season."
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