Former Chelsea striker Jimmy Floyd Hasselbaink hopes Frank Lampard can secure the club's defense.
The Blues are building a fearsome attack, especially with Timo Werner and Hakim Ziyech to sign in the summer.
But their back line is a cause of concern, as they have conceded far too many goals in the Premier League.
"If you look at the team Frank has, with what he has got, he's done brilliant," Hasselbaink told Premier League Productions.
"If you analyse them very, very well they are in the top four but they have had 49 goals against them – that is way too much. 'If you want to compete with the big boys that [defensive record] has to get better significantly.
"If you asked him [Lampard] that's the department that he is really disappointed about. If you see the type of goals that are going in they are easy to defend.
"They're goals that you should defend if you are in the top four or if you are going for the league, those type of goals you should know how to easily deal with."