Chelsea are in the market for a new centre-half signing this summer.
However, Football.London says one of the four current senior centre-backs will need to be sold before any new signing arrives.
And the latest report suggests that one of these three would need to depart in order to make space before potential targets such as Lille's Gabriel Magalhaes or Dayot Upamecano, of RB Leipzig, can move to Stamford Bridge.
Tomori is considered one for the future and highly rated by Lampard, while Christensen struggled with injuries and poor form at the start of the season before regaining his place after the turn of the year.
Zouma has made more appearances than both in the Premier League this term but was wanted by Everton after a successful loan spell at Goodison Park in 2018/19 and is also attracting attention from Tottenham.