The departure of Ferguson's enforcer will leave United with huge boots to fill

Forget Cantona, Beckham and Giggs, no player has been more influential at Old Trafford throughout the last 12 years than Roy Keane, says Glenn Moore

Not that the big opponents, the Arsenals and the Chelseas, bother looking. They know that, however Sir Alex Ferguson rotates his team, if Keane is fit, he plays in the crucial matches. Moreover, even though his body has thousands of miles on the clock, and just as many tackles, he frequently dictates them.

Sometimes he does so without even kicking a ball, or making a tackle. Remember the confrontation with Patrick Vieira in the Highbury tunnel? After Keane had stared down his Arsenal counterpart, United began the game a goal to the good mentally.

No one is spared. Team-mates have felt the wrath of Keane as often as opponents. Anyone who shows signs of coasting, of failing to match his unquenchable desire for success, is in for a glare and a fiery rebuke.

For most of his 11 years at Old Trafford, Keane has been Ferguson's authentic voice on the field. There have been more talented players in that time at the Theatre of Dreams (not a nickname, one suspects, Keane has any time for), there have been more charismatic ones, but no-one, not Eric Cantona, not David Beckham, not Peter Schmeichel, not Ryan Giggs, has been more influential. The precision crossing of Beckham, the willo-the-wisp running of Giggs, the dependable consistency of Gary Neville and the pocket genius of Paul Scholes all played their part in United's domination of the opening nine years of the Premiership but Keane was the engine, the soul, the foundation of the team.

Keane joined United as a 21-year-old in July 1993. Nottingham Forest, who had signed him from Cobh Ramblers in Ireland, had been relegated and Forest's manager, Frank Clark, knew he could hold on to Keane no longer. Word broke that Kenny Dalglish, then managing Blackburn, had been in touch. Ferguson moved swiftly. He later admitted he had been "desperate" to sign Keane for months and was terrified of losing him to Dalglish, as he had Alan Shearer. Initially, Keane was overshadowed by Paul Ince but Ferguson soon realised Keane's arrival had made the "guv'nor" expendable.

Since his £3.75m transfer, Keane has played more than 450 matches for United, compiling an honours board that contains seven Premier League titles, four FA Cups and a regular place in the Professional Footballers' Association's annual team of the season.

There is, of course, one glaring omission on his CV: the European Cup. For Keane's finest hour was also his worst. In Turin, in 1999, Keane was booked with Manchester United trailing Juventus in the European Cup semi-final. The caution meant he would be suspended for the final. It seemed not to matter as United's prospects of reaching the Barcelona showpiece were rapidly receding. Nine years previously, a talented young footballer had suffered a similar fate in a World Cup semi-final. He had burst into tears. Keane merely set to work.

Ferguson recalled: "I did not think I could have a higher opinion of any footballer than I already had of the Irishman but he rose even further in my estimation. The minute he was out of the final he seemed to redouble his efforts to get the team there. It was the most emphatic display of selflessness I have seen on a football field. He inspired all around him. I felt it was an honour to be associated with such a player."

Can anyone still wonder why Ferguson has, in recent years, appeared to have reshaped his team in part to accommodate Keane's increasingly restricted movement? Replacing Keane is, the manager admits, the most demanding aspect of his gradual overhauling of the United team.

But Ferguson must replace him within a season or two because the injuries are coming more frequently and the enforcer can no longer crush opponents beneath his boots 55 times a season.

He does not want to go, not least because he still has a burning need to win the European Cup. Never mind the prawn sandwich brigade, his most coruscating criticism was of team-mates who "were more interested in flash cars and big houses than winning". That scathing review followed the team's exit to Bayer Leverkusen in the 2002 Champions' League semi-finals.

Even last night, as he hinted of leaving United, he railed at the perceived weaknesses of colleagues, demanding they show greater "character".

Keane added: "You need players to stick their chests out and say, 'give me the ball'."

Keane has his faults, not least, as Alf Inge Haaland can testify, a propensity to violence, but no one will ever be able to say he hid on a football field. If the manager who replaces Ferguson is on a hiding to nothing, woe betide the player who has to fill Keane's boots, whenever that is.

Trophies and troubles: Keane's Old Trafford legacy

* Won seven Premiership titles and four FA Cups at United.

* Signed by Brian Clough for Nottingham Forest for £10,000 from Cobh Ramblers in June 1990.

* Signs for United for British transfer record £3.75m in June 1993.

* Picks up first of 11 red cards at United for stamping on Gareth Southgate in FA Cup semi-final against Crystal Palace in April 1995.

* Calls Manchester United fans "prawn sandwich eaters" after Champions' League exit to Bayern Munich in March 2001.

* Walks out on Ireland World Cup squad after row with Mick McCarthy in Saipan, May 2002.

* In October 2002 gets five-game ban and £150,000 FA fine for comments made in his autobiography about a revenge foul on Alf-Inge Haaland in Manchester derby.