Trinity, impressive 3-1 winners at Harrogate Town a week earlier to move seven points clear of the National League North drop-zone, are now just four points outside the relegation places.
Roma told Gainsborough Trinity TV: “It was like after the lord mayor’s show. In the first half we never really got going and it was as poor a performance (I have seen) since I have had the job.
“I said to the lads at half-time we were lucky to come in only 1-0 down.
“We showed a little bit of promise in the second half, but we weren’t at the races today. We need to hold our hands up as a group and say that wasn’t good enough.
“We have had a good little run, but this was one of those days where we didn’t perform.”
Goals in either half by former Football League striker Craig Westcarr meant Alfreton gave departing manager Nicky Law, who is to start a job at Premier League Burnley, the perfect send-off.
Roma added: “Whether they raised their game or not, because obviously it was their manager’s last game, you don’t know, but we should have been able to deal with that.
“It’s hard to put your finger on why (we were so poor) because we did so well last week.
“We are hoping this will just be a one-off and move on.”
Trinity had just one shot on target in the whole 90 minutes to leave Roma puzzled.
“It’s disappointing and frustrating. It is unlike us,” he said.
“Again you can’t put your finger on why and I probably won’t be able to in a week’s time. Everyone had an off day and we weren’t as good as we have been.”