Chelsea manager Thomas Tuchel said his side had not been playing at their highest level since returning from the international break, adding that they needed to wake up if they are to turn around their slump in form.
The reigning European champions were beaten 3-1 by Real Madrid in the first leg of their Champions League quarter-final on Wednesday, after a 4-1 Premier League thrashing by Brentford last weekend.
Chelsea, third in the league on 59 points after 29 matches, travel to 12th-placed Southampton on Saturday.
"Somehow after the international break we had the feeling like we play with 80%, 90% and we hope to get away with it," Tuchel told reporters on Friday. "There are a lot of reasons, which can be that you feel exhausted.
"But you need to overcome it. There is no other way and we need to kind of wake up, or remind ourselves again that the investment physically was not enough in the last two matches. We need to find high-intensity work again."
Tuchel added that it had taken him unusually long to work through his anger at the midweek defeat to Madrid.
"That I stay angry overnight is pretty unusual, yes," Tuchel said. "But the feeling lasted. I watched the match again and I got angry at home, in the middle of the night, and the next morning I watched it again and got angry again.
"You sit there in the middle of the night and the amount of chocolate I needed was immense to go through the match again and compensate. It's not nice ... At some point you press the button to watch it in double speed, just so it passes quicker.
"At some point you have to stop and take a walk into the kitchen or lounge, and calm down."
