The U’s lost their third game in a winless run of four matches when Burton Albion won 2-1 at the Kassam Stadium in a contest of contrasting halves on Saturday.
The visitors sprinted into a 2-0 lead thanks to goals from Dominic Knowles and Zeli Ismail and before the half-hour mark had been reached, Billy Kee had a penalty saved by Ryan Clarke.
United improved in the second half and Ryan Williams pulled a goal back.
They had chances to get on level terms, but the defeat means they have now picked up one point from a possible 12.
It is their worst sequence of the campaign and has seen hopes of automatic promotion recede.
Lewis, who has won two of nine games as caretaker manager, stressed the only way out of the rut was through hard graft.
He said: “It doesn’t matter who the manager is, whether it’s me, Chris Wilder or someone else, some of our players at the moment are not at the top of their form.
“Some of them are senior players and they have to take responsibility – which they do, to be fair, in the dressing room.
“We have all been through it when you are not at your best form and what you do is you work harder.”
The biggest concern for United was the defending.
It was a similar case a week earlier in a 3-0 defeat at Rochdale and Lewis, who felt there were encouraging signs at the other end, was critical of the goals his side conceded.
“The first goal we totally switched off,” he said. “We always say you’re most vulnerable when you’ve got the ball and we definitely switched off.
“The second one we dived in, perhaps we were a bit upset and tense because we had conceded the first.
“We dived in for the penalty as well.”
He added: “To draw positives, as you always must, the second-half performance was really good.
“We created chances, we just need to get back to defending as we have done most of the season.”
Dave Kitson was forced off with an injury in the first half and the initial assessment did not look good for the 34-year-old.
Lewis said: “It was the same hamstring he did before, so I can’t see him playing in the next few weeks.
“It’s bad timing, but we’ve got Dave Connolly.
“The only reason he didn’t start was because he had a bit of the bug which was going around and we didn’t think he could do 90 minutes.
“He came on after about 30 and I thought he was excellent.”