Martial, who joined the Old Trafford club for an initial fee of £36m from Monaco in 2015, has enjoyed an extremely productive start to the 2017-18 campaign, having contributed five goals and six assists in just nine appearances this term.
That haul is all the more impressive given the France international has started just one match in the Premier League this term. In five of his six further league appearances off the bench, he has provided at least one goal or assist after replacing Marcus Rashford, who has been Jose Mourinho's preferred starting option on the left of his attack.
Martial has been the manager's first choice in their opening two Champions League matches with both players in excellent form. But according to Mundo Deportivo, Barcelona are now carefully monitoring the Frenchman's situation and will consider trying to bring him to the Nou Camp should he fail to nail down a regular starting spot over the course of the season.
Despite his healthy return, Martial has managed just 139 minutes of Premier League action, having also struggled to hold down a regular role last season under Mourinho despite his sensational debut campaign in England under Louis van Gaal.
While excelling in a role off the left, Martial has repeatedly insisted in the past he sees himself as a striker. Romelu Lukaku however is United's first choice in that role, however, with Rashford given the nod when the Belgium international was rested in a Carabao Cup victory over Burton Albion. The return of Zlatan Ibrahimovic from a long-tern knee injury in the New Year could further complicate things, with Barcelona ready to take advantage of the situation.
So far United, have paid £43.7m in fees for Martial, having made a further payment of £7.7m after he scored his 25th goal for the club against Burnley last April.
Further clauses dictate United will have to pay another £7.7m should he make 25 appearance for the France national team - he currently has 15 caps – and another £7.7m should he even be nominated for the Ballon d'Or award.