Football League rules prevent Ball, who sees himself as a central defender, from playing again this season for the Millers who are bottom of the Championship and likely to be in League One next term.
So caretaker manager Warne, hoping to welcome back captain Lee Frecklington for the home clash with Aston Villa on Saturday, has told the 21-year-old to show in training that he’s physically hard enough to handle the demands of the position.
“He wants to be a centre-half,” the interim boss said. “He is a great kid. He wants to learn. He is still early in his professional career.
“But to be a centre-half I think you have to be ugly. It helps if you have no teeth, and I just don’t think he’s got that in him yet. I hope the more training he does with us, the more it will develop him as a player.”
Warne played Ball, who has also operated in midfield, at centre-half in his first game in charge, against Burton Albion in December, but dropped him after a poor display and sent him on loan to League One Posh.
The former Spurs youth player clashed with Peterborough manager Grant McCann over team selection after featuring in six matches and returned to South Yorkshire this week.
“There was a fall-out,” Warne confirmed. “Dom has given me his account. I’d like to believe him. There is always 10 per cent fluff.
“We will train him, we will do extra work and he has an opportunity to win over the staff and go into the summer in a good position.”
Frecklington, who has been out with an ankle injury since January 2, has trained fully this week and Warne says the midfield man is in contention for a place in Saturday’s squad.
Striker Carlton Morris should finally arrive in Rotherham after the weekend and Warne already has his eye of a long-term loan deal next year for the Norwich City youngster he likens to former Millers frontman Alex Revell.
Morris, 21, signed for the Millers in January on loan to the end of this season but his move has been delayed by hamstring problems.
“We’ve had this for around six weeks and I keep saying ‘next week’,” Warne said. “You won’t be surprised to hear my answer again is ‘next week’.
“I’ve been told by the manager (Alex Neil) and the player that it’s definitely next week. He signed a new contract there yesterday. Hopefully, I’ll welcome him here on Monday.
“Ideally, he’ll come in, fall in love with the club, do really well and score goals, my charisma will win him over and, potentially, he’ll want to stay and have a season-long loan. That’s the idea.”
“The reason I got him in is because I wanted a striker. I’ve seen him live and on video and I really like him. The people at Norwich really like him.
“He’s a bit like an Alex Revell character. He’s probably not as good in the air but he’s competitive and he’ll run everywhere. He’s pretty aggressive. He’s the sort of player I want here. I want young, athletic people who want to play football.”
Centre-halves Richard Wood (groin) and Joel Ekstrand (knee) are missing for Rotherham tomorrow, while goalkeeper Richard O’Donnell’s is ruled out with a damaged finger, although an initial scan showed no break.
Lewis Price, back in action after injury and illness, will be in goal.