It was the kind of challenge that reflects a mind-set. A ‘head down and tackle anything that moves’ approach from a player making up for lost time. The only problem for Mamadou Sakho was that rather than an opponent, it was the referee Mike Jones who ended up sprawling over his lunge, via a shove from West Bromwich Albion’s Jake Livermore. The crowd guffawed yet Sakho barely acknowledged the official’s tumble, his stare instead fixed on the loose ball as Luka Milivojevic shepherded it away. Then he was up and off, eager for his next involvement.

The scene was The Hawthorns almost a fortnight ago, with Crystal Palace defending a narrow lead and their French loanee rejoicing in the ferocity of battle. His performance that afternoon, refusing to wilt in his duels with Salomón Rondón, was inspirational, that of a man adamant his team would not be breached, and all the more remarkable given it was only his second first-team outing since April.

Friday marks an anniversary, though not one Sakho will cherish. It is a year since a routine drug test after a fine draw at Old Trafford, a result which secured the passage of Jürgen Klopp’s side into a Europa League quarter-final and effectively sparked the end of his Liverpool career.

So much since has been traumatic. He has been banned then reprieved, deprived of a place at Euro 2016 in his home country, exiled by his club, and finally cast aside to a team whose sole aim is to ward off the considerable threat of relegation. “The lift he has given us defensively has been exceptionally good,” said Sam Allardyce, who took Sakho to Palace and will build his backline around him against Watford on Saturday. “He has settled into the environment here extremely well and worked hard to get himself fit. Now he is showing what a good defender he is.”

But as much as the 27-year-old is at ease alongside compatriots in Yohan Cabaye and Mathieu Flamini, Loïc Rémy and Steve Mandanda, or friends such as Christian Benteke, the script he had envisaged has been torn up. None of this is how it was supposed to be.

Some of the setbacks suffered over the past year have been self-inflicted, but they all stem from that drug test. The urine sample, scrutinised at a World Anti-Doping Agency-accredited laboratory in Cologne, tested positive for Higenamine, an anti-asthmatic which can also be used as a fat burner. Sakho never denied taking the pill and after liaising with Liverpool voluntarily made himself unavailable while his legal team, led by Mike Morgan, a specialist in this field, determined how to proceed. His last appearance was as a goalscorer in a 4-0 victory over Everton. Less than a week earlier he had plundered his side’s equaliser in that jaw-dropping comeback against Borussia Dortmund.

The timing of his absence was dismal. Sakho had become a stalwart of this team, his partnership with Dejan Lovren suddenly blossoming and his status among the support raised. If there was frustration the player had opted to take a product that had not been prescribed by Liverpool’s medical staff – evidence of an individualism that does not always sit easily with Klopp, but also of fitness concerns which had occasionally surfaced at his first club, Paris Saint-Germain – then the management were still incandescent to have lost him.

They recognised the team were weaker in his absence. The provisional 30-day ban imposed by Uefa on 28 April eventually ruled him out of the last eight games of the season, including the Europa League final in Basel where his influence might have been decisive. Sevilla prevailed after a dominant second half. Sakho, meanwhile, was preparing to take on the authorities.

Mamadou Sakho sends the referee Mike Jones flying at the Hawthorns Photograph: Andrew Boyers/Reuters

His defence, which would eventually prove successful, had hinged upon Morgan demonstrating to Uefa’s satisfaction that the science that could reasonably lead Wada to class Higenamine as a banned substance – as a beta2-agonist – was not robust. But by the time Uefa had cleared Sakho of wrongdoing, and ordered its own investigation into whether the substance should be banned, he had missed the run-in, Basel and selection for Didier Deschamps France squad at Euro 2016. “Everything he had been doing had been geared towards representing his country in a home tournament,” said Sakho’s wife, Majda, in an interview last month. “Personally, I went through so many emotions: shock, bewilderment, frustration as well. And anger.”

Sakho’s lawyer is still exploring the possibility of legal action, most likely against Wada, which chose not to appeal against Uefa’s decision to absolve his client, and the player has only briefly addressed the disappointment around the ban. “Whatever I go on to do in my career, it cannot replace what I missed out on with Liverpool and France,” he had said at the end of January in an appearance on Canal Football Club. “I had wanted so much to wear France’s colours in a tournament taking place on home soil. Seeing my dreams snatched away from me three weeks from being realised … that was hard. But I knew I wasn’t a cheat.”

He spent time in Senegal and Ivory Coast during the tournament, plunging himself into work for his charity, AMSAK, whose initiatives address education to healthcare and water sanitation in impoverished communities. He visited a juvenile prison, and a school for deaf and dumb students, in Abidjan. He would later take deprived families from Liverpool on a trip to Disneyland Paris. But he could not cut himself off entirely from the tournament where he should have been adding to his 28 caps. “We saw him at the team hotel before one of the games, and it was as if he was our No1 fan,” said Noël le Graët, the president of the French Football Federation. “He was hurting so badly not to be involved, but he still wished the team luck. That shows the class of the man.”

The ramifications of the ban lingered beyond the summer. Even with a live-in personal trainer who had apparently put him through his paces three times a day, the club were distinctly unimpressed by Sakho’s fitness levels upon his return to Melwood in July. The focus is regularly drawn to him interrupting Klopp in an interview with Liverpool’s in-house TV channel while on a visit to Alcatraz in pre-season as evidence of indiscipline, but the manager could see the funny side of that exchange. What infuriated him more was Sakho’s tardiness in reporting to the training ground on the day the team had flown to the United States. When he subsequently missed a morning session with a fitness coach, as well as a team meal, he was sanctioned and sent back to Merseyside. His apology fell on deaf ears.

Mamadou Sakho interrupts Jürgen Klopp during Liverpool’s tour of Alcatraz last summer. The manager saw the funny side but sent the defender home over other matters. Photograph: YouTube

Klopp demands total professionalism in training and preparation, and evidence of an eagerness to improve. It is not as if he craves a side of automatons. Indeed, he has dealt well with the behaviour of other mavericks, most notably this year after Roberto Firmino’s drink-driving charge. Yet the Brazilian’s commitment in training, or industry in matches, is rarely in doubt. Players have to graft under the German and as soon as standards drop, so does the axe. Those pre-season errors of judgment, even in a player still coming to terms with how the glorious denouement to his best campaign for Liverpool had been so cruelly snatched away, counted against him. There was an achilles injury to endure and, in the autumn, the bitterness was exposed with a series of exasperated messages posted on Snapchat in the small hours of 24 September. Klopp saw Sakho’s outburst – an insistence he had been fit for three weeks and the fans “deserve to know the truth” – as a betrayal of the team ethic as if designed to heap pressure on his replacement, Ragnar Klavan, when the side confronted Hull later that day.

It effectively cast him back into the wilderness for over four months. There were outings for Michael Beales’ under-23s, occasions which “allowed me to gain some rhythm from playing again, and to pass on my experience to the younger players”. But a player contracted until 2020 clearly had no future at Liverpool and having rejected loan offers in the summer, he was more receptive in January. So after Southampton dithered and Lyon turned up their nose, Palace were the beneficiaries, agreeing to cover the player’s wage package and a hefty loan fee in the region of £2m.

Inevitably, he did not arrive match fit. There was concern for a while that pushing him too hard might ping a hamstring, and successive defeats were endured with Sakho on the bench. Yet once thrown into the fray, he excelled. His two outings, against a shot-shy Middlesbrough and an awkward West Brom, were won without a goal shipped. To put that into context, Palace had kept two clean sheets in their previous 30 Premier League games. That rather haphazard style, forever bordering on clumsiness, is deceptive. There is an agility to Sakho’s game, and those instincts to block, tackle or stifle in the penalty area are as sharp as ever. For all Klopp’s attempts at deflection, his excellent form has not gone unnoticed back on Merseyside, where the suspicion remains that Liverpool are missing their most naturally gifted defender.

Palace’s newfound stinginess is not down to the Frenchman alone – the efforts of Milivojevic and the fact Allardyce’s tactical demands are finally sinking in have also been key – but he has been the poster boy for the mini-revival. “He’s a rock, so solid and at the same time, so composed,” said Wilfried Zaha. “If you make the movement he’ll make an angle for you and find you with his pass. That’s massive for us. We’re way more solid now. We’ve gone back to basics and if you do those right, the rest will follow.”

The team have inherited that same leader who made his mark in PSG’s junior ranks, the kid from Barbès in the 18e arrondissement who captained Paul Le Guen’s first team at 17. But he is eager to prove himself all over again, rebuilding that reputation. Although Sakho may not be used to struggling at the foot of the division, the fighter in him will relish the scraps ahead. He would regularly whip up the dressing room at Liverpool pre-match, a player going to war. It was why he was recognised as a big-game player, the kind who would raise his performance for the most daunting of contests. Palace, with collisions with the top six among their remaining 11 games and only a point above the bottom three, needed a figure around whom they could rally. “Mama is already a big part of what we are trying to achieve here,” said Allardyce.

If he succeeds in hauling them to safety, a player who still has his best years ahead will have proved his class and other clubs will be interested. This is his chance to show his pedigree is intact, and that miserable year is behind him.