
Barton caught Etuhu high on the thigh with his studs, leading the midfielder to eyeball him and prompting much ill-feeling as the players left the field at half-time.
Referee Martin Atkinson did not even give a foul for the challenge, which occurred significantly after the ball had gone.
But in a statement released to Sky Sports News, Barton, who was told Monday he will not face sanction over the incident, tried to explain himself.
"There was no malice in it, nor was it premeditated," he said. "If I caused him any injury, then I apologise.
"Things happen in the heat of a derby game. It was my third game back and I'm still a bit rusty.
"If I was fully match fit the situation would have been avoided."
The Football Association, meanwhile, explained their logic in not charging Barton.
A spokesman said: "The Football Association can only take action in the case of incidents that are not seen by officials.
"Whilst it is clear that the officials did not see the full extent of the incident, they did see players coming together and to take any further action would be tantamount to re-refereeing the game and this would be contrary to the laws of the game."