The Real Valladolid striker agreed to move to the Welsh capital in February, signing a pre-contract agreement with the club.
Cardiff were relegated from to the Championship following a 3-0 defeat to Newcastle United nine days ago, casting Guerra's prospective switch into doubt.
However, Cardiff manager Ole Gunnar Solskjaer insists that the deal will still go ahead.
"He's (Guerra) still joining us yes," Solskjaer told Wales Online . "It's a watertight contract all the papers are signed.
"He still needs to do his medical, but he will come and do that this month when the Spanish league has finished. That's all sorted though. After that he joins us for preseason."
The 32-year-old netted twice in Valladolid's 4-3 loss to Real Betis at the weekend, taking his tally for the season to 16.
Solskjaer added: "He scored a couple of goals on the weekend. He's a very exciting player”
