Chelsea's 3-0 Champions League last-16 first-leg home defeat by Bayern Munich showed "there is a lot of work to be done", says manager Frank Lampard.
They were well beaten, with Serge Gnabry scoring twice and Robert Lewandowski scoring a third.
"It was a harsh lesson," said Lampard, who also called it a "reality check".
"There's no point looking at anything other than being real on where we are, fighting for the top four because we want to improve in the summer."
The Blues, who appointed their all-time top scorer Lampard as manager last summer, have not reached the Champions League quarter-finals since 2013-14.
They were under a transfer ban last summer and have blooded several youngsters this season.
However, they had as many players in their 30s as they did players aged under 25 - three of each - in their line-up against Bayern.
"We weren't confident on the ball," said former Derby boss Lampard. "That was my biggest disappointment.
"We want to play here. We've shown that all season but today we went away from that. That can happen sometimes in an intense atmosphere.
"We haven't been fighting in the knockout stages and getting to the back end of it in a few years."
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Three of the Bayern starting line-up from the 2012 final - which Chelsea won on penalties with Lampard in the team - played on Tuesday.
"We saw a lot of quality in their team. I played against Thomas Muller, Jerome Boateng and Manuel Neuer in the 2012 final. Robert Lewandowski and David Alaba have been there a long time.
"There's a lot to their team we have to respect. Today was a clear show there's a lot of work to be done. I've felt that all the way through since taking the job.
"They [Chelsea players] need to use it as a positive effect. They won't feel like it this evening but what they need to do is understand the levels of the Champions League when you get to the knockout stage.
"A lot of players haven't played in it before. When you have an eye-opener like tonight, the only answer - young or old - is to say I'm not going to look at any part of the team other than myself. Who was I up against? How do I feel I played against him?"
Lampard added the second leg would be difficult - no team have ever overcome such a deficit from a home leg - but refused to rule out a famous comeback.
"We have to go and play with pride and not consider too much the scoreline," he said. "We know we're 3-0 down going to Munich. It's pretty clear we're in a bad position going into the second leg. A show of character is all we can do."