The Dutchman acknowledged that winning the title is improbable from the position Barcelona now find themselves in, with Real Madrid and Atletico Madrid both ahead of them in the table and with games to play.
"Our aim was to win all three games and hope that the others would lose points," Koeman said. "But now it's difficult.
"Games last for 90 minutes and we lost a lot in the second half. They [Levante] caused problems and we didn't defend well.
"It's very disappointing. It's impossible to understand what happened. It's difficult to explain it."
Ousmane Dembele played with a lot of defensively responsibility on the night, but ended the evening with a goal and an assist to his name.
"He caused a lot of danger on his wing and he was very good defensively," Koeman said of the winger. "It was more extreme for him to play at wing-back. His counterattacking was devastating."