The Telegraph says Shaw played against Ukraine, Denmark and Italy at Euro 2020 with the injury.
The United full-back was a standout performer for Gareth Southgate's Three Lions this summer, scoring one goal and supplying three assists in six matches.
It turns out the defender faced Ukraine in the quarter-finals, Denmark in the last four and Italy in the finale at Wembley with fractured ribs.
The report reveals that the 26-year-old sustained the injury in the last-16 victory over bitter rivals Germany.
United are waiting for Shaw to return from his summer break to find out the extent of the damage.
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