First up, I am not a professional and this is only my opinion.
I am a little confused by your reference to #20 as I can't see number 20?
My opinion: Blue #15 thinks he can win the ball and launches himself across the grass, sliding off his feet . He realises that he is not going to be able to make the tackle (or that his teammate will?) so pulls his legs in an attempt to pull out of the challenge, but because he is off his feet he is out of control and he continues into the back of the green player.
Certainly careless = foul
Reckless? Yes as he acts with disregard to the danger or consequences of an opponent = yellow card
Uses excessive force? Don't think he deliberately did, as he tried to pull out of the challenge, but the force on the green player was excessive, but it could be argued that the force used was excessive and endangered the opponent (we can't judge on player intent, as we have no way of knowing a players intentions) which equals a red card (and subsequent ban.)
For me: Yellow card, but wouldn't argue with anyone who wanted to give him a red.
As soon as you come off your feet your are a danger to yourself and others as you have lost control of your body.