Ferguson and Keane's relationship has been frosty to say the least since the midfielder left Old Trafford under a cloud in 2005 - going on to sign for Celtic - but it is of Keane's coaching credentials that the Scot has been the most scathing.
Keane has been involved in frontline management twice since retiring from playing eight years ago. His first season at Sunderland in 2006/7 could not have gone much better, Keane taking over in late August after a poor start in the Championship and leading them to the title.
His second campaign, this time in the Barclays Premier League, was unsurprisingly much tougher, Sunderland losing 7-1 to Everton and flirting with relegation for a while. They eventually finished 15th after a late flurry of victories.
Keane lasted until December in his third and final campaign in charge at the Stadium of Light, standing down with Sunderland again in relegation trouble. Results had been poor, although Keane later revealed there had been differences of opinion with club owner Ellis Short and chairman Niall Quinn.
In April 2009, Keane was appointed manager of Championship side Ipswich Town, winning their last two games of the season, but when the new season got under way, Ipswich failed to record a win in their first 14 matches. Their form would be better in the second half of the season but after too many draws they finished 15th, well out of contention for a shot at promotion.
The next season was not much better and Keane was sacked in January after 21 months in charge.
In his autobiography, Ferguson was critical of Keane's managerial style, believing his former captain was "impatient" and looked for quick fixes by signing players in bulk rather than taking the time to build a team. "The press used to see him [Keane] as a quasi-manager [during his playing days] because of his winning appetite and the way he drove the team on," wrote Ferguson. "They would ask me all the time: "Do you think Roy Keane will be a manager?" As his career in coaching developed, it became apparent that he needed to spend money to achieve results. He was always looking to buy players. I don't feel Roy had the patience to build a team."
Keane bought six players on deadline day three days after arriving at Sunderland, then added four in the January of the same season. He also recruited heavily after winning promotion, signing, among others, Craig Gordon for £9m, Michael Chopra for £5m and Kenwyne Jones for £6m. He also spent around £6.5m in his first summer at Ipswich.
Keane, though, could respond simply to Ferguson's criticism by saying that his outlay was a fraction of that spent at the top end of the Premier League - £7m for Bebe anyone? - while it is hardly unique for a manager at an ambitious Championship club to push the boat out a bit to try to attain promotion. There would have been few at Sunderland bemoaning the wage bill when the league trophy was back in the cabinet, while in his second summer at Ipswich he actually brought in more than he spent.
All this, of course, supposes that Keane wants the Celtic job. Now assistant to Martin O'Neill at the Republic of Ireland national team, the 42-year-old said recently that he was in no rush to get away, although the right position may tempt him.
"People keep asking me about other jobs," he said. "In my quiet moments, I obviously want to go back to being a manager. But I have got a job and I am very happy in my job. I am really appreciating the opportunity I have at the moment. I was at a match and was up in the stand thinking, 'I would love to have a team'. And then I got back in my car and spoke to Martin and was thinking I was delighted to be working with him, Seamus [McDonagh, the goalkeeping coach] and the other staff. So you have to be careful what you wish for. But what happens further down the road …"