"They could make an offer for Hernan after Argentina's match," said Hidalgo.
Crespo will be Chelsea's ninth major signing of the summer following the arrivals of Glen Johnson, Wayne Bridge, Joe Cole, Duff, Geremi, Adrian Mutu, Veron and Alexei Smertin.
They could still be joined by Real Madrid's Claude Makelele now that he has ended his strike at the Spanish club.