Chelsea boss Jose Mourinho has instructed his scouts to keep a close eye on 21-year-old Marseille midfielder Imbula, who has impressed since joining the Ligue 1 side from Guingamp over the summer.
And Porto centre-back Mangala has been a long-term target for Mourinho, although the Blues hope they can reach a deal for less than the Frenchman's £37m release clause.
Imbula is valued at £25m by his club, which means Chelsea face a potential maximum cost of £62m for the pair.
Chelsea have a fight on their hands then to secure one of the targets, let alone both. The report claims Mourinho has switched his attention to Imbula after being priced out of a move for Juventus star Paul Pogba.
The Italian champions are thought to be looking for a fee in the region of £40m for Pogba and while Chelsea are still interested, it's believed Imbula is the better value option.
Mangala has emerged as one of the most wanted centre-backs in European football over the course of the last 12 months.
He impressed in Porto's title-winning campaign last season, and in the Champions League, and Chelsea scouts reportedly believe he could replace John Terry at the club one day.