Burton fell to second-half goals from Potter and Danny Rose after having defensive general Ian Sharps sent off by referee Michael Bull for kicking out at Dave Kitson, who escaped with a yellow card despite retaliating.
Lewis said: "Alfie hadn't trained all week and only came in on Friday, but he has got that ability to change games.
"We always feel that late on when players are getting tired that he's got that ability to go past people and cause problems.
"I think he scored a very similar goal here two years ago.
"We've had three tough games away from home - and Burton is one of the toughest places to come.
"But we work hard defensively, we've got a good shape about us and we've got players who are good on the counter.
"At times when you're playing against 10 men the biggest problem defensively is concentration, but there was always someone there to cover.
"When we looked at the fixtures, we said the first eight or 10 games were really tough, so we're happy where we are at the moment, but we're not getting carried away."
Potter cashed in Burton's defensive indecision to waltz past three opponents and score with a deflected shot in the 50th minute.
Second-placed Oxford sealed the three points five minutes from the end when Rose forced home Ryan Williams's cross from close range to sentence Burton to only their fourth home league defeat since the start of last season.