Aaron Martin
Aaron Martin has made 39 senior appearances for Espanyol to date Denis Doyle/Getty Images

Manchester United appear to have joined the likes of Manchester City, Barcelona and Real Madrid in continuing to keep a watchful eye on Espanyol's Aaron Martin as manager Jose Mourinho once again ponders the addition of a new left-back.

ESPN report that the Premier League title contenders dispatched scouts to run the rule over the Spanish Under-21 international during Espanyol's goalless La Liga draw with Levante on Friday night (13 October), a match in which he featured for the full 90 minutes.

The same source reports that United, who could soon cut ties with the disappointing Luke Shaw and have generally opted to alternate between Ashley Young, Matteo Darmian and Daley Blind at left-back so far this season, have also asked to be kept abreast of any developments regarding Martin amid suggestions that he could bid farewell to RCDE Stadium as early as January.

United are believed to have watched Martin in the past, with scout Toni Lima sent to observe both the Catalan and defensive midfielder David Lopez during a 1-0 victory over Alaves in April.

City and Real's interest in one of Spanish football's breakout young stars was established earlier this year and, according to The Mail, the former also had scouts in attendance against Levante.

Free-scoring City made a concerted effort to significantly overhaul their ageing full-back options over the summer, offloading the likes of Bacary Sagna, Pablo Zabaleta, Aleksandar Kolarov and Gael Clichy and lavishing over £100m ($131.6m) on the trio of Kyle Walker, Benjamin Mendy and Danilo.

However, they could dip into the market once again when the transfer window re-opens for business in the New Year with Mendy rupturing his cruciate ligament in only his fifth outing for the club against Crystal Palace in September. He is expected to be sidelined until around the time of the Champions League semi-finals in April and could feasibly miss the remainder of the season.

Real were previously said to view Martin as a long-term replacement for Fabio Coentrao, who joined Sporting CP on loan in July having long since become surplus to requirements at the Santiago Bernabeu. Los Blancos eventually raided rivals Atletico Madrid to secure a back-up to first-choice left-back Marcelo, signing Theo Hernandez to a six-year deal.

Barcelona have also been linked with Martin, with RAC 1 claiming in June that they had contacted neighbours Espanyol to express their interest in a player who has not looked back since being handed his senior debut by Quique Sanchez Flores back in October 2016. Lucas Digne currently serves as the deputy to Jordi Alba, who is currently sidelined after suffering a thigh injury on the eve of a European tie against Olympiacos.

Espanyol evidently recognise the talent they have in Martin and in January promptly handed him a new five-and-a-half year contract. That deal, which runs until the summer of 2022, is said to contain a €30m (£26.8m) release clause that is scheduled to increase to €40m from 30 June 2018.