Sacko became the third signing since Garry Monk took over as head coach and could be followed by veteran Osman, who has been released by Everton.
The England-capped midfielder is being targeted by Leeds as they continue to re-shape the squad for the new season and would be available on a free transfer.
The 35-year-old’s Goodison Park career was brought to an end after 12 years as a first teamer when he was not offered a new contract this summer.
He is keen to continue in the Premier League but could be open to dropping to the Championship with United looking to bring in some experience to help young midfielders at the club like Lewis Cook, Alex Mowatt and Kalvin Phillips.
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Sacko completed his loan move from Sporting Lisbon yesterday and is training with the first team squad in Ireland.
The 22-year-old can occupy a number of attacking roles and could play in next week’s friendlies with Shelbourne and Shamrock Rovers.
Sacko, who is capped by his France at under 20s level, spent the second half of the last campaign on loan in France with Ligue 2 side Sochaux, scoring twice from 14 appearances. He originally moved to Portugal in 2014 from Bordeaux, where his professional career began and has made over 50 appearances for Sporting’s ‘B’ team.
Leeds midfielder Casper Sloth, meanwhile, has returned to Denmark to join Superliga side Aalborg BK for an undisclosed fee.
The 24-year-old, a summer 2014 signing from homeland outfit Aarhus, played just 14 games in all competitions during his two-year stay with United.
United will host Serie A side Atalanta at Elland Road for a pre-season friendly on Saturday, July 30 (3pm kick-off).
The Italian side, who finished 13th in their league last season, will provide Leeds’ final friendly opposition of the summer ahead of the new Sky Bet Championship campaign that gets under way the following weekend.
United’s other pre-season friendlies are away against Shelbourne on Wednesday, July 13 (7pm); Shamrock Rovers on Saturday, July 16 (3pm); Guiseley on Friday, July 22 (7.45pm) and Peterborough United on Saturday, July 23 (3pm).