Cardiff boss Ole Gunnar Solskjaer is keen to sign "at least one" central defender before the end of August.
One target could be Bordeaux's Senegal international Lamine Sane, in spite of reports that his club have offered him a new contract.
With Chile midfielder Gary Medel's £10m transfer to Inter Milan expected to be completed soon, Solskjaer is keen to recruit.
"There will be at least one centre back coming in to boost the squad," he said.
Cardiff kick off their season in the Championship against Blackburn at Ewood Park on Friday without central defenders Ben Turner (ankle injury) and Juan Cala (suspended) and with no replacement as yet for last season's captain Steven Caulker who has joined Queen's Park Rangers.
"We've sold Steven Caulker and we've not brought anyone in yet because we haven't found the right one," added Solskjaer.
"But I need to cover my bases."
Solskjaer no longer has the option of midfielder Medel dropping into the defence with the Chilean's protracted transfer to Inter nearing completion.
"In principle the deal is done [and] it will be announced very soon," said the Norwegian.
The Bluebirds are understood to have made enquiries about Sane, 27, who has one year left on his current contract in France.
Cardiff could face competition from Fulham who are also thought to be keen to sign the defensive midfielder.