The Red Devils shot stopper spent the second half of last season on loan at Aston Villa.
Middlesbrough, West Brom, Leicester, Huddersfield as well as Villa are all ready to raid for the 24-year-old, according to the Daily Mail.
Villa boss Steve Bruce was impressed with Johnstone and would like United to lend him his services once again.
Huddersfield and Boro are keen on a permanent deal being done though.
David Wagner is looking for a new keeper for the Terriers as Liverpool want to keep Danny Ward at Anfield next season.
Johnstone's representatives at James Grant Sports explained: “Sam has a number of options and is considering them all at this moment in time.”
United won’t be rushed into any decision with the future of David De Gea and youngster Joel Pereira undecided.
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Johnstone played 21 times in the Championship last season, keeping eight clean sheets in the process.
United are expected to be busy in the transfer window as Jose Mourinho prepares his side for a return to the Champions League.
They have already signed Benfica defender Victor Lindelof for £31m.
