The 28-year-old took up the chance to join Nottingham Forest on loan last week after being completely frozen out by Benitez this season.
Colback was not used on a single occasion by United in the first six months of the campaign, but he was given a 90-minute run-out in Forest's 2-0 defeat to Fulham at the weekend.
Speaking after making his first outing of a frustrating campaign, the ex-Sunderland midfielder is quoted by the Newcastle Chronicle as saying: "It's been difficult. I think there's ways that things can be managed, and to be honest there's been no respect whatsoever shown my way, but that's the game I suppose.
"I've had to keep my head down, I've had to work hard for myself, I've done that and I'm thankful the manager here at Forest has given me the chance. I'm not sure [why I wasn't given respect], I'm not the one who should be answering that - you'll have to ask the boss back home.
"But it's one of them things. I'm big enough not to be too upset about it, I've been in the game a long time and seen a lot of things. I think the disappointing thing is the way it's been dealt with. I've been treated like a number more than a human being.
"But there's nothing I can do about that, he made his choice and I'm not really looking back at that one. There's nothing I think I could have done differently. It was his choice, he said he had too many numbers; I was named in the squad of 25 but still couldn't manage to train with the first team, and that's the way it goes."
Colback, used a number of times at Under-23s level this term, is understood to have rejected moves to Hull City and Sunderland last summer.