The decisive moments came in the 88th minute when the U’s, trailing Rotherham United 2-1, had two great chances to equalise.
Both James Constable and Deane Smalley had efforts from close-range saved by goalkeeper Scott Shearer and within seconds the Millers broke to wrap up a 3-1 win.
Lewis – who was always going to do the post-match interviews instead of boss Chris Wilder – rued the visitors’ inability to make the most of their chances.
The United assistant manager said: “We’re hugely disappointed. It’s hard to put into words. “I think you look at that moment with a minute to go when we had two really good chances to score and within ten seconds it’s 3-1, which was a major blow. “I think we did more than enough to get a result.
“We had a couple of chances to go 2-0 up as well, which would have made a different game of it.”
He added: “The boys (Constable and Smalley) are pretty honest and both said afterwards they should have scored.”
The defeat extended the side’s losing slump to six successive league games.
Lewis admitted it was tough for everyone to take, but stuck to the belief one positive result could turn their form around.
“The players are a really determined group – I’m standing here sick as a pig, but they are exactly the same,” he said.
“You’re worried because you want to get that next win, but I’ve been here long enough to know that one win can end up being five wins on the trot. “Then it’s a completely different picture, as it was at the start of the season. “Yes, that win needs to come pretty quickly now, we know that. No-one likes losing games.”