The U's were beaten 1-0 at home against rivals Peterborough United on Saturday to leave their League One survival hopes in the balance.
Cambridge are 23rd and 13 points adrift of safety, with their head coach fully aware of the immense challenge they face to stay up.
Harris, who returned for a second spell in February, spoke to BBC Cambridge Sport afterwards and didn't hold back with his thoughts.
"Absolutely gutted and feel absolutely robbed, I'll be honest, it's the best way to sum it up. 8,000 Cambridge fans should feel exactly the same.
"Look, I can't talk anymore positively about the group and their attitude, and demand to want to win, and application to go out and play on the pitch, and play really well at times.
READ MORE ON THE EFL
'Minor miracle' - EFL club legend makes instant impact as manager with nine-year first
Frank Lampard extends incredible Coventry record that stretches back 40 years
"Sometimes playing really well against the ball, getting pressure on, turning the ball over, creating chances, getting bodies in the box, just everything about it was so good, apart from the most important part and that's putting the ball in the goal, and stopping it at the other end.
"The fans have travelled to Blackpool and Wigan, will have come away really pleased with the performance they saw and the attitude of the players, we lost both games. We've seen it again today, we were the better team in so many ways.
"Yes, Peterborough made a lot of passes, sideways, backwards, didn't break us down too often. We created chance after chance after chance, didn't have the killer moment in front of goal. Why? Because it's about the culture, it's about the culture.
"I can't change that today, I couldn't change it during the week, I can't change it next week coming up. I said before, I can change it in transfer windows, I can change it from the first day of pre-season, but it has to change.
Most read in EFL
'Minor miracle' - EFL club legend makes instant impact as manager with nine-year first
Frank Lampard extends incredible Coventry record that stretches back 40 years
EFL club sack manager as ex-Premier League player is instantly named interim boss
EFL boss bans stars from shopping, bowling and Costa Coffee in strict new rules
"8,000 people don't deserve to come and pay hard-earned money, bring their kids to football, to go and buy burgers and hot dogs, and train tickets, to come and watch their team get beat, it's not fair. I don't want to be the manager of this football club that sees that, I won't have that at this football club but I can't change it today.
"Am I proud of the players and the way they played, they gave me a performance? One million per cent, absolutely every player played their part in that. But we have to be clinical in both boxes. We don't have a clean sheet mentality at one end and at the other end, we're not brave enough in front of goal."
Harris then added: "Well I've just said to the players, the last nine games are more important for them. They've got to think about not just their futures at this football club, but references that they're going to see for relegation on their CV, the players.
"Look, we don't want to look down the barrel and say, 'Look, it's very unlikely we're going to stay in the division'. Yeah, let's just be honest about it, of course it is, it was unlikely when I walked in the building, when you're eight points adrift.
"Ten points adrift, it is unlikely, you don't pick up whatever we've got, 30 points in 46 games prior to me walking in and then expect just to go and pick up 20 points in ten games for example. It just doesn't happen in football, or very, very rarely happens in football.
"So what I want to see is players that want to play for their futures, that want to play for this football club, that want to represent this football club.
"I need that application and attitude, but I need a better culture, I need better standards at the training ground, that's from staff and from players."
Harris then commented on the players playing for their futures: "It's their livelihoods. It's their job, but it's also their passion and their hobby as well, and they have to show me, they have to show us, show the football industry that they're worthy of a place at football clubs.
"I like working with the group, I like their application in so much, but application isn't always just enough. It's accepted in the sense of, 'Good lad, good attitude', you want to work with good people like that.
"But you've got to have the killer instinct as well, in both boxes, you've got to have the drive and the determination. That's what sets levels, that's what levels is in football.
"Levels is players that play in the National League and League Two because they're missing something. Sometimes it's because they're not quick enough, sometimes it's they're not technical enough, sometimes they just haven't got the bloody heart for it.
"So that's what I'm seeing in the group at the moment and that's what I question about this group, have they got the passion and the drive and the hunger to win games of football? Well clearly not. Thank you gentlemen."
Harris then walked away from the interview as he faces the ongoing task of keeping Cambridge in League One of another season.
The former Millwall boss got off to an excellent start on his return with back-to-back wins against Stockport County and Crawley Town.
Read More on talkSPORT
Peter Crouch loses £7.8k after being forced into Guinness Cheltenham bet
Lewis Hamilton opens up bravely on fatherhood and reveals interest he's 'given up'
Take a closer look at the 16,000-seater EFL stadium delayed because of 'poorly prepared application'
However, Cambridge have lost all four of their fixtures since then and look set to be one of the four teams that will go down.
Barnsley are up next before tough tests against promotion hopefuls Wrexham, Charlton Athletic, Huddersfield Town and Birmingham City.