Coventry fought back from 3-0 down to draw 3-3 and take the tie to penalties, which they ultimately lost.
Rashford was eventually substituted by manager Erik ten Hag and the Manchester Evening News says Coventry's players were surprised by how he faded in the second-half.
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The Sky Blues felt they could run off the United attacker and were amazed by his lethargy on such a big day.
Indeed, one Coventry player expressed understanding with those United fans frustrated with Rashford this season.