Gianfranco Zola says Chelsea are "not the perfect team" and don't create enough chances given the possession they have.
Zola is assistant to head coach Maurizio Sarri, who admitted following Wednesday's 0-0 draw with Southampton that Chelsea have not been good enough recently in the "last 20 metres".
While many Blues fans are happy to attribute those problems to the form of struggling striker Alvaro Morata, Zola feels it is due to wider-ranging issues.
Speaking ahead of Saturday's FA Cup third-round tie at home to Nottingham Forest, Zola said: "I can tell you in the past Barcelona had problems in the last 20 metres, Bayern [Munich], [Manchester] City, everyone suffers when there is no space up there.
"This is the big issue we have, for the amount of time we have the ball we don't create enough chances. It is part of the process, we are not the perfect team.
"Southampton wasn't easy, when you have five games in your legs and it's the sixth it is not easy. We want to pay more attention to this [lack of chances]."
Morata, a reported £70 million ($88m) signing from Real Madrid in July 2017, has struggled to convince at Stamford Bridge and has five goals in 16 league appearances this season.
Asked to evaluate his current woes, Zola said: "It is mainly confidence.
"I have been a striker and you get into loops when you look for goals and they don't come. It happens. Alvaro has to understand to be important for the club. The way we play it helps the front players to score, you have to keep working and the goals will come."
Zola says Willian, Olivier Giroud and Pedro won't be available, which will leave some space for younger players. Callum Hudson-Odoi is fit again and available. He adds that there will be rotation to the side. #CHEFOR— Chelsea FC (@ChelseaFC) January 4, 2019
Sarri will ring the changes for the Forest game with Eden Hazard set to be rested and in-demand winger Callum Hudson-Odoi likely to start.
Zola added: "We have some players who cannot play, like Pedro, Willian and [Olivier] Giroud. It will leave space for others. We will rotate a little.
"Callum is back in full training, he is certainly available and I would very much like it if he is going to play."