The Sky Blues were linked with a deadline day swoop for both players, but baulked at the Portuguese champions’ £48m demands.
However, their interest is thought to remain strong, which has led to talk of a fresh raid once the season finishes.
But Chelsea are now ready to offer them opposition in the battle.
Blues chief Jose Mourinho was busy during last month’s transfer window, yet he is still thought to be keen to draft in more players to build a new look Chelsea side.
Mangala is the more highly-rated of the double act, and is widely thought of as one of Europe’s finest emerging defenders.
He has a release clause of around £38m, but Porto may be willing to sell for a fee below this in the summer.
Fernando on the other hand will be a more cost-effective signing, with the rangy midfielder due to be a free agent in the summer.
City are likely to make moves for both, but Mourinho is confident that his relationship with the club may aid his cause.