"Believe me, it was really hard for me," Nani said in a statement on his agent's official website. "That moment, I just wanted to disappear, and I did just that.
"Since I left the stadium right away, I couldn't be with my team at the end.
"But next Tuesday I will be at the club to redeem myself for that mistake and tell all of them that I'm sorry for what I have done and apologise for my behaviour."
United ended up coasting to a 4-1 win against West Ham on Saturday, even though they were down to ten men for the entire second period, but Sir Alex Ferguson's side still require a win at Wigan next Sunday to clinch the title.
Nani added: "I publicly want to tell [everyone] that I'm deeply sorry, firstly to my team-mates for the extra effort over 60 minutes, secondly to my coaches and finally to United's supporters, who have been great to me.
"I know my act was heedless but I would also like to say that I was provoked and hit. This is not an excuse but this was exactly what happened.
"I am sorry for what I have done and I must thank my team-mates for being able to secure a win of major importance for us."