Was shocked to see a red given at the time and watching it back I still don't think it was a red.
Bertrand was tackling to his right-hand side and with his left leg, therefore effectively running parallel with the player - try doing this and keeping your left foot on the floor the entire time. It is extremely difficult! To face forward and swing your left leg around to the right to tackle a ball it means you lose balance and cannot keep the foot down. As a result, Bertrand's boot comes off the top of the ball and into the guy's leg.
Can you tell I think it was slightly harsh?!