Emanuel Pogatetz feels Middlesbrough should be concerned about their current position in the Premier League table, while he has also reiterated the importance of keeping Stewart Downing and Tuncay Sanli.
Boro slipped down to 14th after being convincingly beaten by Fulham on Saturday, with Jimmy Bullard, Danny Murphy and Clint Dempsey on target in a 3-0 victory for Roy Hodgson's side.
There was some controversy about the penalty that led to Fulham's second goal but Pogatetz admits that Boro did not deserve anything from the game.
"It was one of the worst performances of the season," the Austrian said in the Daily Mirror.
"We were a bit unlucky to go 2-0 behind with the penalty, but we shouldn't hide behind that decision because we deserved to lose.
"It worries me because that was just not good enough.
"I am always disappointed when we lose, but this was different to losing at Hull because of a refereeing decision.
"We didn't want it as much as Fulham and that's why I'm hurt."
The result left Boro without a win in their last six games and just two points above the relegation zone.
Pogatetz has called for his team-mates to respond in the right manner but he is worried about Boro losing some of their best players.
Downing and Tuncay have both been linked with moves away from The Riverside and Pogatetz does not want to see them leave.
"It's very important we bounce back against Everton on Boxing Day because the table is very tight and we are at the wrong end of it," he explained.
"We can't afford to lose Downing or Tuncay. We have to keep hold of our best players - I mean, we have sold a few players over the last couple of years and didn't bring in a lot of replacements.
"This is a very young, inexperienced squad who have a bright future, but the likes of Downing and Tuncay are vital to our team because they are both international players with experience.
"I still believe in the squad we have got, but some of the players have to understand our standards should be higher."