Luiz, on-loan from Juventus, is being linked with a move to Chelsea.
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But Dyche explained: "He's ill. He's got what's going around - the flu sort of situation.
"He came in and spoke to the doctor.
"We said we had to get him out of the camp because we had such a long injury list when we got here and we don't want illness to creep in as well.
"When you are travelling on planes and all the rest of it, it spreads like wildfire. Hopefully he will recover quickly.”