Barcelona have made an official bid for Chelsea defender David Luiz according to Sky sources. The Spanish heavyweights have been linked with the Brazil international all summer and are understood to have finally made their interest formal on Wednesday. But Blues boss Jose Mourinho has insisted the 26-year-old is central to his plans and the bid is expected to be rejected. Speaking after his side's 2-0 pre-season win over AC Milan Mourinho said: "David has come back with us fantastically and we have to use his quality. "He has big self-confidence and self-esteem especially when he comes up from the back with the ball."