Having spent much of the season inside the top seven, the Cod Army's play-off hopes hang by a thread after being taken apart by Keith Hill's men.
Alexander shared his frustrations with the players after the match and his views remained the same having cooled off over the weekend before subjecting them to a DVD playback of the contest.
"It was a very bad day at the office," he said. "I spoke straight after the game when my emotions were high but two days later my opinion of the game hasn't changed one bit.
"It was terrible. We've all watched the game together, the whole team, and they saw it for themselves how bad it was.
"It wasn't misinterpretation of what we saw, they could see it for themselves. The whole lack of anything resembling a professional performance was shocking to see.
"What we're looking for now is a response to that because we have to show we're not as bad as that week-in, week-out, which we're definitely not.
"We just can't afford to have any more performances like that at all."
Asked whether the display was his worst since succeeding Micky Mellon at the Highbury Stadium in December, Alexander added: "Yes, without a doubt. Definitely.
"We've had a couple that we've been angry about and we've let ourselves down in parts of games but I thought we let ourselves down for 94 minutes on Saturday."