Chelsea great Ron Harris and supporters chiefs have told fans to forget about Frank Lampard for 90 minutes on Saturday and back the Blues.
Lampard, Chelsea's record goalscorer, returns to Stamford Bridge with Manchester City bidding to cut the five-point deficit to Jose Mourinho's men at the top of the Barclays Premier League standings.
The 36-year-old spent 13 years at Chelsea and insisted he would never play for another Premier League club.
So playing for City, on a one-season deal before moving to New York City FC in Major League Soccer, tarnishes his status in the eyes of some Chelsea fans.
"There will be some for him and some against," Harris, Chelsea's record appearance holder with 795, told Press Association Sport.
"They should support Chelsea. Frank's a terrific lad and served Chelsea ever so well.
"(But) if he was to score the winning goal for Manchester City, it might blow up."
Didier Drogba, now in his second spell with Chelsea, was heralded by all sides of the ground when he returned with Galatasaray in the Champions League last season, but Lampard is unlikely to be afforded such a welcome.
Chelsea Supporters' Trust chair Tim Rolls told Press Association Sport: "I don't think anybody is going to boo him. Lampard is respected, but I think there's a frustration he said one thing and did another.
"Assuming he comes on as a substitute, he'll get applause and he might have a few chant his name, but I don't think it will be like Drogba.
"Life goes on. It doesn't particularly wind me up, but a number of supporters are wound up.
"If you fast forward 10 years, I don't think people will remember that. I think they'll remember the trophies he won and the goals he scored.
"Lampard is a sideshow really for Saturday. If Chelsea win we're eight points clear with 15 games to go. It's an absolutely fundamental game for our season. I'd hope the support will get behind the team."
Chelsea were on course to beat City at the Etihad Stadium in September until Lampard equalised with five minutes remaining.
Rolls added: "At Man City, when he came on, before he'd scored, he got a lot of support from Chelsea fans. My thought was we should be supporting the team, not an ex-player.
"He scored, City fans started chanting his name and some of our fans started chanting his name, during the match. That was a little bit strange."