Carlo Ancelotti felt Everton deserved to take a point from a dour game with West Ham at Goodison.
But a late Tomas Soucek goal condemned the Blues to a first league defeat since late November and thwarted hopes of moving up to second in the Premier League table.
Everton could muster only two shots on target but Ancelotti believes his side should have emerged with a point.
"It was not a good performance," he said.
"We didn't deserve to win the game but we didn't deserve to lose. It was a lucky deflection and we lost the game.
"We are disappointed and frustrated but, honestly, we missed one point from this game and we must try to catch this point in the next game [Wolves away].
"The performance was not at the top. Compared to the last game, we had more of the ball but we were not able to manage the ball as we wanted, with speed and with quality in the final third. We didn't have a lot of opportunities in the game. West Ham defended really well."
Everton remain fourth in the table as they now turn their focus to next weekend's FA Cup tie with Rotherham.
"Our momentum was good," Ancelotti said.
"We had difficult games but we won. We had a good run which has now stopped but we must keep the belief high and keep looking forward to the next game.
"We are now in a position that in eight days we hope to put some of the players who have not been able to train with us in this last period, in a better condition. We have to look forward and keep our belief."
