While clubs across the global footballing landscape seek to strengthen their ranks during the notoriously challenging January transfer market, a growing question mark is hanging over the heads of several soon-to-be free agents.

Players whose contracts are due to run out at the end of the 2023-24 season are now able to engage in talks over pre-contract agreements with foreign clubs, whereby teams can shake hands on a free transfer in the summer rather than trying to prise them away from their current stomping grounds in January for a nominal fee.

For some clubs, it may simply be a case of triggering a short-term extension or finalising protracted negotiations over a renewal in order to avoid losing one of their star performers. For outcasts with one eye on the exit door, agents will be working around the clock to find them a new home.

Here, Sports Mole takes a closer look at some of the esteemed players who have now entered the final six months of their existing deals and are free to speak with foreign teams at their leisure.

GOALKEEPERS

With goalkeeping hierarchies almost always being established in the summer transfer window, noteworthy moves in between the sticks should be few and far between in the winter months, although a handful of established number ones could be without a goal to guard come the end of the campaign.

Among them are Napoli's title-winning shot-stopper Alex Meret, who only extended his deal with the reigning Serie A champions in October 2022, but the 26-year-old - who is still the Partenopei's undisputed first-choice - is running down the final few months of that short-term renewal.

Meret could spend another 15 years barking orders to his backline and would surely be inundated with offers if he was to enter the free agent market - Roma and Inter Milan have been credited with an interest - although his deal does include an option until 2025, which Napoli president Aurelio de Laurentiis would surely be inclined to trigger.

There is no such clause in the contract of Wolfsburg's experienced Belgian goalkeeper Koen Casteels, though, and the 31-year-old is already confirmed to be departing the club in 2024, albeit in the summer rather than midway through the campaign.

Plenty of revered veterans are also facing uncertainty over their long - or short-term - futures, including Paris Saint-Germain's Keylor Navas, Anderlecht's Kasper Schmeichel, Celtic's Joe Hart and Roma's Rui Patricio, all of whom still have plenty of life left in their hands.

Schmeichel is even believed to have caught the eye of Premier League basement battlers Sheffield United, whose current number one Wes Foderingham also falls into this category, while two Arsenal youngsters in Karl Hein and Arthur Okonkwo could see their Emirates stays end this summer too.

DEFENDERS

Despite what Manchester United initially communicated, former World Cup winner and four-time Champions League champion Raphael Varane does not have 18 months left to run on his deal, as the terms that the Frenchman penned in 2021 instead run for three seasons with the option of a further 12 months.

Varane was hitherto hopeful that Man United would choose to retain his services until 2025, but it is now believed that the Red Devils have opted not to trigger that clause, thus forcing the 30-year-old to scope out new - or old - opportunities amid links with former clubs Real Madrid and Lens.

A January departure has not been ruled out for Varane, who is one of four Man United defenders in the pre-contract boat alongside Aaron Wan-Bissaka, Victor Lindelof and Jonny Evans, but the former two are set to have their deals extended while Evans has also been given a vote of confidence by Erik ten Hag.

The Old Trafford quartet are just some of a plethora of Premier League defenders who are at risk of waving goodbye to their current clubs, a list which also includes Fulham pairing Kenny Tete and Tosin Adarabioyo, Tottenham Hotspur outcast Eric Dier, Liverpool's Joel Matip and Newcastle United stalwarts Fabian Schar and Jamaal Lascelles.

A duo of West Ham United right-backs in Vladimir Coufal and Ben Johnson also find themselves in the last six months of their terms - the former is allegedly unhappy with the Irons' plans to trigger his 12-month extension rather than offer him a fresh deal - while Bournemouth would do well to cash in on Lloyd Kelly amid reported interest from Liverpool and Tottenham.

Two celebrated defenders of this generation - Sevilla's Sergio Ramos and Borussia Dortmund's Mats Hummels - could be on the move in January or in six months' time too, while speculation is also rife over Atletico Madrid's Mario Hermoso, Lille's Tiago Djalo and Real Madrid's recently-appointed captain Nacho Fernandez.

MIDFIELDERS

Following the departure of Casemiro to Manchester United two summers ago, the passing of the Real Madrid midfield torch could very well be completed in 2024, as Toni Kroos and Luka Modric are now in the latter stages of their respective contracts.

The pair's influence in the Blancos team has waned since Jude Bellingham, Federico Valverde, Eduardo Camavinga and Aurelien Tchouameni arrived on the scene, and while Kroos is supposedly ready to ask for another renewal, Modric could close his glittering Bernabeu chapter.

Kroos and Modric may be in the twilight of their careers, but a few engine room enforcers in peak condition could also be available on free transfers in the shapes of Napoli's Piotr Zielinski, Juventus' Adrien Rabiot - who could still sign a new long-term agreement in Turin - and Real Betis' Guido Rodriguez.

Two other renowned European lynchpins in the shape of injury-plagued Liverpool man Thiago Alcantara and Arsenal's Jorginho could soon leave the Premier League behind, as might West Ham United's Pablo Fornals, Tottenham Hotspur's Ivan Perisic and Manchester United's Hannibal Mejbri, although a loan move appears more likely for the latter.

It remains to be seen if Jorginho's fellow Gunner Mohamed Elneny is blessed with another short-term extension too, and elsewhere in Europe, Borussia Dortmund legend Marco Reus is only half a year away from a possible emotional farewell from the Westfalenstadion after a 12-year stint.

ATTACKERS

Several drawn-out sagas later, the long-term future of Paris Saint-Germain headline-maker Kylian Mbappe is no closer to being resolved, and the French champions are now powerless to prevent the 25-year-old from engaging in negotiations with European clubs at his leisure.

Two years after rejecting Real Madrid's nine-figure sum, PSG allegedly did their utmost to force Mbappe out of the door in the summer after being informed that he would not activate his one-year extension, going as far as to ostracise him from the main training group for a short period of time.

Mbappe has since returned to his prolific ways as he continues to run down his deal, but Real Madrid will not wait forever and have apparently set a deadline of mid-January for the Frenchman - who has also been tipped to make an audacious move to Liverpool - to make a final decision on his next career move.

A couple of Mbappe's compatriots in Anthony Martial (Manchester United) and Wissam Ben Yedder (Monaco) see their terms expire in the summer too, as does Porto's Mehdi Taremi, who has been tipped to replace Martial at Old Trafford.

Failed Real Madrid attacker Luka Jovic has struggled to reignite his career with AC Milan too and may be searching for a new club once the campaign concludes, but on the other hand, Angel Di Maria has been an astute addition to the Benfica ranks and may have played his way into an extension.

The situations of West Ham United's Michail Antonio and Brighton & Hove Albion's Danny Welbeck are less clear, though, and it remains to be seen whether Milan's Olivier Giroud will continue to don the Rossoneri colours too.