Off contract at the end of the season, Willian has been requesting a three-year deal, and has no intention of agreeing the club's offer of two years.
"It would blow my mind if they just let him go for free," Johnson told the Mirror. "You try to replace him nowadays you're spending 40, 50, 60million.
"He's been, other than [Christian] Pulisic, probably their best player."
Meanwhile, Johnson feels Jorginho's lack of playing time does not necessarily mean Lampard wants to get shot of the 29-year-old.
"He's a decent player, obviously Lamps know him better than I do," Johnson added. "These clubs have got big squads so not everyone can play so I wouldn't assume he wants to get rid of him, he's just rotating the squad and seeing what the other players can do."