Yeah you don't need physics for this :P Assuming you're using the 8 directional behavior..
Give your player a variable, lets name it "hurt".
When the player gets hit, set "hurt" to 10, set his deceleration to 100 or something else very low, set his angle of motion to the angle between himself and the enemy using angle() and finally set his speed to however fast you want him to be knocked back.
Now if that player variable is greater than 0 then subtract, say, 30*dt. When it's lower or equal to 0, activate your player event group and reset his deceleration.