Chelsea have two targets: Frenkie de Jong and Bernardo Silva. However both of them are very tough ones. Frenkie de Jong is likely to be kept by Barcelona this summer but there is still a slight chance for a possible transfer. As for Bernardo Silva, Manchester City aren't thinking about selling him now. He is important for his team and they wouldn't even want to sell him to a rival team I guess. Therefore Chelsea had better have some backup plans.
For Frenkie de Jong Barcelona, it has confirmed not to sell this season, differently with true Silva the opportunity to be purchased is far more open to now, depending on the agreement offered by Chelsea, in football there is no term not to sell to the opposing club, because the player which is released or purchased is very dependent on the agreement, advantages and demands of the player itself, but the backup plan is needed by Chelsea, if the two players still survive in the old club, then they have the choice of other players to be purchased
Frenkie de Jong is not interested in leaving Barcelona right now. Barcelona also needs de Jong to strengthen the midfield. So Chelsea has to hope to get Frenkie de Jong into the team. Bernardo Silva is a key player for Manchester City. Guardiola is unlikely to decide to sell Silva. Chelsea coach prefers players whom Chelsea would not be able to recruit. Chelsea could not add any such big star player to the team this season.