It was Albion's second loss to United in five days, having gone down 3-2 at the AMEX on Saturday.
And Potter believes his side were close once again to the Premier League giants.
He said: "It was a tight game until the second goal, or Dean Henderson's save at 1-0 was key. Then they bring on Rashford and Pogba which made life difficult.
"The first goal is not a good one, we have to do better there, it's as simple as that.
"We were in the game, 3-0 is a bit harsh."
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