Cottagers boss Martin Jol had hoped that Ruiz would be available for the busy Christmas schedule, but the Costa Rican will not return before the end of the year after aggravating the problem in training.
"He came back on Monday then he had a setback and he will be out for at least four weeks," Jol said. "It is the same injury. He was in the final stage of his rehabilitation and he aggravated his hamstring muscle again.
"He has had intensive screening and that highlighted some problems. He had this hamstring injury before, now we've found out it is a structural thing.
"Hopefully we have found a solution. We sent him to Germany with our people and Doctor [Hans-Wilhelm] Muller-Wohlfahrt is a specialist I know from my time there.
"Hopefully he will be fine in four weeks now but of course it is a big disappointment. Bryan is a vital player for us and every time he plays. Good luck to him and he will be back after Christmas."
Jol also revealed that defender Sascha Riether is a doubt for Monday's clash against Newcastle after he picked up a knee injury.
However, the problem was described as a "minor knee strain", and Jol expects the German to return to training on Saturday.
Midfielders Alex Kacaniklic and Simon Davies have already resumed training following injury and could be available for Monday's encounter at Craven Cottage, live on Sky Sports.
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