A broken bone or a torn muscle might have been easier to come to terms with.

An illness that robs Darren Fletcher of his strength, his energy, his vitality, seems to be the cruelest fate. It strikes at his defining quality, the vigour that carried him from Mayfield, a council estate near Dalkeith, into the very heart of Manchester United's esteem. Even in dealing with the ulcerative colitis that has caused him to take an extended break from the game, Fletcher's instinctive refusal to succumb compromised his health as well as his fitness. He would not concede until he had no choice.

Fletcher stayed in Manchester when Scotland departed for their penultimate Euro 2012 qualifier against Liechtenstein last October, with the explanation that he was suffering from tonsillitis. Yet he flew out the day before the game, proclaiming his fitness and his willingness to be involved for his country. This sense of self-sacrifice has been a recurring trait and if it appears more grave now, Fletcher's commitment to the national team, his insistence on turning up for every game, whatever the circumstances, must also be reassessed as an act of devotion.

Fletcher is a vital player for Scotland, but in many ways a more important leader. His attitude, his professionalism, his dedication, all have become established as the core values of the squad under Craig Levein, a manager who reveres him. Roy Keane made a similar impression on his team-mates at Old Trafford, setting standards for timekeeping, seriousness, loyalty, and level of play – Fletcher once recalled being excoriated by the United captain for giving the ball away in training – but the Irishman expressed that authority through his wrath.

Fletcher is more subtle, but no less fiercely adamant. He influences people through his quiet, almost sombre resolve and the dedication that has been a source of encouragement throughout his career. As a teenager, Fletcher fractured the same metatarsal five times, but never allowed the recurring injury to disrupt his progress, and he now has a metal pin in his foot. He declined contract offers from Rangers and Celtic as a youth, preferring to wait until he was 16 to join Manchester United, even although the move was never guaranteed; he was convinced that he would be taken on at Old Trafford, and then thrive.

His self-belief is uncompromising and the characteristic that Sir Alex Ferguson would have immediately cherished. They discussed his contract during a game of snooker at the manager's Cheshire home, after he invited Fletcher and his father, Bobby, down, and United colleagues still joke to Fletcher about Ferguson being his dad, or grandad. There is no room for favouritism at United, though, and Fletcher succeeded because of his mental strength. He admits that others in his age group were talented passers, dribblers, headers or goalscorers, but he is the only one to have graduated to the first-team.

Fletcher is a midfielder of dynamism, of relentless application, but also shrewdness. Ferguson unleashes him upon teams and there is something defiant about the way this thin, willowy, pale blond-haired figure harasses opponents, stealing from them their composure and poise as he bears down on them with his rangy stride. Fletcher had to overcome his appearance, though, since United fans were sceptical, not least because he was breaking into a midfield that contained Paul Scholes and Keane.

Fletcher is an amalgam of modest qualities and his contribution has come to be treasured, particular when suspension denied him a place in the 2009 Champions League final and United had no terrier to let loose on the Barcelona midfield. His absence only emphasised his value to the team, and Ferguson's judgment that no player better epitomises the attitude and the temperament for the major occasion.

His relationship with Scotland has been just as complex. When he was first called up to the national squad in 2003, one tabloid ran the headline: "At last, we find a Becks of our own". His status as a United player came to be a burden, though, since Scotland fans expected him to be all the players that he is not: the creator, the scorer, the enforcer. Fletcher has always been a more understated combination of features.

He responded by trying to do too much, hitting "World Cup passes" and "Roy of the Rovers stuff", as he recently admitted. Fletcher has now come to terms with his Scotland role, settling into his natural demeanour of understated command. The evening after he scored the goal against Lithuania that secured Scotland's place in the Euro 2004 play-offs, he went back to his parents' home and took his three sisters out 10-pin bowling.

His family were agricultural workers, and Fletcher used to help his father during the school holidays in what he later described as "back-breaking" work. When he left for England, it was with the resounding advice that this was his chance and he should make the most of it. Fletcher shunned nights out as a result and he remains teetotal.

Just as Levein has come to laud the player, so too do Scotland fans. It is a belated acclaim, but more lasting because he earned it through his efforts. At United, Fletcher might be seen as the spirit of Ferguson's enduring commitment to the cause, as if his indefatigability was a reflection of his manager's attitude to the game. He is less clearly defined as a Scotland player, because the quality of the team around him has been so variable, but perhaps it is enough to recognise his willingness to be that vulnerable, to push himself to the forefront when others could not.

At this time of uncertainty, when he cannot even know what the future will hold, Fletcher will still be best served by the traits he has relied upon as a footballer: his steadfastness, his conviction, his courage.