The Cobblers sat deep in npower League Two’s bottom half a month ago, but they head to the Kassam Stadium in ninth, just three points from third place.
The transformation has come after collecting 13 points from their last five games.
United are down in 18th after going four games without a win, a run extended by a 2-2 draw at Barnet on Tuesday night.
But Lewis feels Northampton’s experience shows how rapidly the U’s fortunes could switch – if they can find some form.
United’s assistant manager said: “We’re heading the right way, we just need a couple of results and it’s amazing how it can change. “Look at Northampton, they were 17th and they’ve won three games on the trot and all of a sudden they’re one point off the play-offs – it’s that quick to change.”
He added: “You can see the (United) players out on the training ground and they’re absolutely terrific. “They’re disappointed, we’re disappointed (with the results), but if we keep working hard we’re pretty sure that it will change.
“We can’t harp on about it and worry about it, we just have to believe in what we’re doing.”
United will be forced to make one change in the back four, with Johnny Mullins recalled by Rotherham United.
With Andy Whing unlikely to be moved from right back, it is likely to be a straight choice between the squad’s two fit specialist centre backs.
Lewis said: “I think it’s disappointing to see Johnny go because he performed so well, but straight away we’ve got Michael Raynes and Harry Worley vying for a place.
“It’s not like we’ve lost a player and we haven’t got anyone to play at centre half, so it’s an opportunity for somebody else.”
Lee Cox is available to come back into midfield after serving a suspension, while left back Liam Davis is in contention after recovering from a hamstring injury.
For the first time this season, Northampton will be without full back Joe Widdowson, who is suspended after picking up his fifth booking of the season in Tuesday’s 3-0 home win over Morecambe.
As well as in-form striker Adebayo Akinfenwa, United must be wary of Ben Tozer’s long throws, which have set up 11 goals already this season.
TODAY’S SQUADS Oxford Utd: from Clarke, Whing, Raynes, Wright, Davis, Heslop, Cox, Leven, Rigg, Potter, Constable, Pittman, Marsh, Chap-man, Forster-Caskey, Capaldi, O’Brien, Worley, Boateng, Brown.
Northampton: from Nicholls, East, Langmead, Carlisle, Tozer, Robinson, Hornby, Harding, Demontagnac, Hackett, Akinfenwa, Snedker, Charles, Huws, Platt, Mukendi, Moult.