Record says United have been scouting the left-back for much of this season.
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Benfica signed Dahl permanently from Roma over the summer after taking him on-loan last season.
And it's suggested United scouts have been impressed by Dahl and a summer move is now being considered.
Dahl's contract with Benfica extends until the summer of 2029.
The defender, 23, was not selected for Sweden's successful World Cup playoff campaign this week.
Dahl has made two senior appearances for Sweden so far.