Calls for action grew louder on Saturday with a 3-0 defeat at Chesterfield, which saw Tom Newey and Michael Raynes sent off late on.
The U’s ended the game with eight men, as Johnny Mullins was stretchered off with a foot injury after all the substitutes had been used.
It left United winless in six games as their place in the Sky Bet League Two play-offs increasingly comes under threat.
The away fans on Saturday chanted for chairman Ian Lenagan to make a decision, seven weeks after Chris Wilder left the club.
United’s chairman was watching the game, but declined requests to comment on the situation.
Lewis, whose record as caretaker manager is two wins from 11 games, was only able to reiterate that his aims were on what happens on the pitch.
“I can’t keep stressing it enough, it’s not about me,” he said.
“People can have their opinions. I’m happy doing it, I want us to win the next game and that’s all that matters.”
United’s odds of taking something from the game lengthened significantly following a terrible start.
David Hunt and Deane Smalley gifted the league leaders a 2-0 lead, and Lewis was concerned by individual errors.
He said: “It’s got nothing to do with formation, or shape, or personnel, we just gave two awful goals away and it puts yourself on the back foot.
“If you’re playing top of the league and you’re 2-0 down after 15 minutes it’s always going to be difficult.”
Raynes was dismissed for two clumsy challenges, but Newey received a straight red card for a reckless tackle.
Lewis was disappointed and now faces a defensive headache for Friday’s home game against Hartlepool United, which Mullins is also likely to miss.
Lewis said: “It’s a poor challenge by Tommy and as an experienced player it shouldn’t happen.
“It’s frustration and people lacking in confidence.”