Real Madrid have announced an agreement to sign highly-rated teenage defender Jesus Vallejo from Real Zaragoza.
Aston Villa have continued their spending spree after securing the services of Rudy Gestede from Blackburn on a five-year contract.
Talks are reportedly continuing between the two clubs to agree terms on a deal for the Morocco international, with United keen to add to their thin defensive options.
Louis van Gaal handed youngster Tyler Blackett his first-team debut in the 2-1 Premier League defeat by Swansea on Saturday because of the absence of injured centre-back Jonny Evans.
Benatia was linked with a move to Manchester City earlier in the summer, but they baulked at a fee of around £30m and went on to sign Eliaquim Mangala from Porto last week instead.
United are yet to replace Rio Ferdinand and Nemanja Vidic from last season's squad and looked vulnerable at the back during their opening day reverse at Old Trafford.
Benatia was born in France but has played most of his career in Serie A, having joined Udinese from Clermont in 2010 and then moved to Roma in the summer of 2013.
As well as being a solid defender, Benatia has an eye for goal as he scored five in 33 appearances in his first season with Roma as they finished second in Italy's top division.