Winger Watkins has returned to Pittodrie on a two-year deal following an injury-disrupted loan spell at the club last season, while midfielder Longstaff has joined on a season-long loan from Newcastle.
Ryan Hedges remains sidelined by a hamstring problem until after the international break, while Andrew Considine is out until after Christmas after undergoing cruciate ligament surgery. Connor McLennan is hopeful of returning following a hamstring strain.
Ross County manager Malky Mackay could hand a debut to Jack Baldwin after the 28-year-old centre-back moved to the Highlands earlier this week following his departure from Bristol Rovers.
Jack Burroughs misses out after suffering an ankle injury ahead of last weekend's defeat by Rangers and the on-loan Coventry midfielder is expected to remain sidelined until late September.
On-loan Southampton full-back Jake Vokins still faces at least a couple of months out with the foot injury he sustained earlier in the season.
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