Republic of Ireland striker Anthony Stokes was one of three players dropped after being late for the departure of the team bus for Saturday’s 2-0 win at Barnsley.
Swedish winger Tobias Hysen, son of former Liverpool defender Glenn, and Hungary keeper Marton Fulop, were also ordered to remain on Wearside and train with the juniors.
It was the second time in two weeks that 18-year-old Dubliner Stokes, a £2m January signing from Arsenal, had been left out for disciplinary reasons.
Manager Keane said: “It’s been spelt out to the players on numerous occasions about the importance of being on time.
“The players are well aware. We finished training on Friday at 12.30 and we left at 4.15. If you can’t get home, get your bag organised, get in your nice car and be on time, there’s something drastically wrong.
“I played with players for 12 or 13 years who were never late once. I’ve been at this club about six months and there have been a number of players who were late for training and for team meetings.
“I will give players the benefit of the doubt if there’s a genuine reason.
“I can be quite a forgiving person. But when it happens more than once or twice, there’s something seriously wrong.
‘‘There has been that lacklustre approach, especially when I first got the job, but you have to draw the line somewhere — and the line has been drawn. Until the penny drops, I will be constantly doing what’s best for the club and what I have done was best for the club, even if we had lost. I wouldn’t have lost any sleep about my decision.
“I had to take this stance for the likes of Grant Leadbitter, who came in and scored, and Ross Wallace, Tommy Miller, our young keeper Trevor Carson, and Stephen Wright and Kenny Cunningham, who have come back from injury. Daryl Murphy is also back on the scene.
“They’re good lads and they train well and I’d rather have those players in my team. That’s the road I am going down. If you train well and train hard, you are going to get your chance, but if you mess about you will suffer. It’s very straight forward.”
Keane added: “The team has to come first and nothing will interfere with that.
‘‘Ultimately, there’s a good chance the players who missed out would have played.
“But I don’t believe in dragging things out. They have had their punishment, we move on and they will be involved in the home game with Stoke on Tuesday.”
On the eve of the trip to Barnsley, Stokes had revealed how Keane was “putting the frighteners” on the players.
Said the Dubliner: “He sent us to army camps and white-water rafting, so we are wonderingwhat he will do next. He will probably have us parachuting or something like that, it wouldn’t surprise us. That unpredictability is good.
“You don’t know what’s going to happen with Roy Keane from day to day. That’s a good thing. For me, personally, it’s been great to work under him in the game Leadbitter struck in the 66th minute after being set up by substitute Murphy, and David Connolly scored a superb second — his 11th goal of the season — in injury time.