The former Ipswich defender was taken off on a stretcher at QPR with a thigh problem and looked in agony as he was carried off the field.
Diallo looked as if he had settled in well when he played against Ipswich, Stockport and Rangers but went down innocuously and could be set for a month or two on the sidelines.
Meanwhile, Frankie Simek is reportedly suffering from a knee injury and could be a doubt for the clash at Leicester on Saturday.
The American defender has played well since his move from Arsenal but new signing Maurice Ross will be waiting in the wings to replace him.