Manchester United reportedly dispatched scouts to Portugal to watch Porto's 2-2 draw with Vitoria Guimaraes with defensive duo Alex Sandro and Danilo understood to the subject of their attention.
The Brazilian pair joined Porto in the summer of 2011 after they paid Santos £20m to to fend off interest from domestic rivals Benfica and secure the services of the talented duo. Both players quickly secured places in the Portuguese giants' starting line-up, with Sandro excelling at left back and Danilo replicating his compatriot's good form on the right flank.
According to Portuguese newspaper A Bola, they may be tempted to maker another move together at the end of the season, with Manchester United and Everton closely monitoring their development.
The Premier League holders are reportedly preparing a massive clear-out at the end of the season to avoid another disappointing campaign and in addition to a centre-back to replace Nemanja Vidic, they are also keen to sign two defenders for both flanks.
Danilo would act as a back-up for Rafael Da Silva, who is currently the only natural right-back in the squad, with the versatile Chris Smalling and winger Antonio Valencia covering in his absence this season.
United are also desperately looking for a left-back to replace Patrice Evra, whose contract is due to expire at the end of the season with little sign of an extension being signed as of yet.
Alexander Buttner has failed to prove himself as a consistent back-up for the French international this season and United have reportedly looked into signing Sandro, although Real Madrid's Fabio Coentrao, Atletico's Filipe Luis and Southampton youngster Luke Shaw remain other possible options.
Sandro, 23, have been capped by Brazil six times and has featured in 19 Primeira Liga games for Porto this season, grabbing three assists.
Danilo, 22, has also played six games with the Brazilian national team while this season he has scored two goals and grabbed three assists in 20 domestic league appearances for the reigning Portuguese champions.