The Old Trafford club are on the hunt for a new centre-back this summer and have made the England international their top target.
However, the Mail Online reports that they could shift their attention to Senegal international Koulibaly after falling short with two bids for £80 million-rated Maguire.
The Napoli defender has been frequently linked with the Red Devils and could be targeted as executive vice-chairman Ed Woodward works on strengthening Ole Gunnar Solskjaer’s squad before deadline day on August 8.
Woodward will not join the squad on their pre-season tour for the first time since taking the job six years ago, as he looks to step up transfer dealings.
He has little more than a fortnight to complete any remaining deals, which could also include a bid for Newcastle midfielder Sean Longstaff.
The Magpies have set a £50 million price tag on the youngster, but Woodward believes he is worth £20m and is expected to table a slightly improved offer.
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