Downing injured his knee against Birmingham last week and suffered with swelling follow the game. The problem re-occurred after the Charlton game on Sunday, so Boro are seeking the advice of a specialist.
The Boro winger has been forced to withdraw from the England U-21 games against Wales and Germany scheduled for Friday and next Tuesday.
Follow his visit to the specialist Downing will undergo an operation that could sideline him for three months, depending on the extent of the damage.