Scouts from Manchester United and Arsenal were in the crowd for Sporting Lisbon’s 3-0 defeat at Vitoria de Guimaraes last weekend.
Midfielder William Carvalho is believed to be one of the main reasons for their presence.
The Portugal international, aged 22, is a long-term transfer target for both clubs. United scouted the anchorman on a near-weekly basis under David Moyes last season, while Arsenal reportedly lodged a bid for him in the summer transfer window.
He appears to still be on the radars of both the Premier League sides, with Portuguese newspaper A Bola reporting the the Red Devils and the Gunners were both watching Carvalho during the Vitoria game.
Carvalho has a release clause of £35m in his Sporting contract, but he is likely to be available for much less.
Arsenal reportedly bid £23m in the summer, with the only stumbling block being the amount they were prepared to pay up-front.
