The Blues finished in fifth spot in the Premier League table last term to miss out on a Champions League spot for the second time in three seasons.
The west London side have failed to sign any players in the summer transfer window to raise questions about their ability to challenge next term.
Chelsea FC are facing interest from FC Barcelona in Willian after the Brazilian grew frustrated with life under Italian head coach Antonio Conte.
Hazard, meanwhile, has often been linked with a move to Champions League winners Real Madrid over the past few seasons.
Sunday Times reporter Castles believes Chelsea FC are prepared for the worst case scenario involving Hazard and Willian this summer.
“I know Chelsea have for several months gone into the window preparing for the possibility they lose Hazard and Willian,” Castles told the Transfer Window podcast.
“It isn’t actively wanting to sell either player but knowing they are in a potentially comprisable position.”
Chelsea FC won the Premier League title in 2016-17 after the Blues finished seven points ahead of their closest title challengers.
The Blues were 1-0 winners against Chelsea FC in the FA Cup final at Wembley following Hazard’s winner from the penalty spot.
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