The 22-year-old Irishman started Derby's first five games of the Championship season, but is set to miss all of this month after dislocating his shoulder in the 1-1 draw with Ipswich Town last time out.
Although initially expected to be fit again in September, Simpson has now said that a return next month is more realistic.
"He has still got a bit of discomfort in the area around the shoulder, which he is bound to have, and we are looking at a four-week plan to get him back involved with us," Simpson told reporters.
"That probably won't be any time this month but we are hopeful he might be available for the first game of next month if everything goes according to the rehabilitation plan the medical department have given him."
Next up for Derby in the Championship is a trip to East Midlands rivals and early pacesetters Nottingham Forest on Sunday.