Gallas kicked the back of United winger Nani's legs during the latter stages of the game, with the altercation seemingly not witnessed by referee Alan Wiley.
Ferguson believes Gallas should have been sent off and intimated that Wiley did not want to show another red card after dismissing Emmanuel Eboue earlier in the game.
"Obviously, a lot of clubs will be disappointed because of their players who have suffered from a respective view of incidents," said Ferguson on Sky Sports News.
"I think, for Alan Wiley, maybe it was a tough game in the second half for him and he'd probably had enough and was quite happy to put a marker under it and get a rest for the remainder of the week.
"It was obviously very difficult for him in the second half and there could have been three or four red cards for Arsenal.
"The important thing was that everyone has been made aware of the incident, that's the thing we don't need to answer to. We're happy to leave it now."