Lomas could only watch in disbelief as his side went behind to an early strike from Derby's Jake Buxton.
The Rams' Scottish midfielder Craig Bryson worsened his mood with a hat-trick, with substitute Mason Bennett completing the rout late on, while a red card for Millwall's Shane Lowry did not help.
Certainly it would have been a brave person to tell Lomas to consider a 75th-minute strike from newcomer Martyn Waghorn, on loan from Leicester, as some sort of consolation in a 5-1 loss.
Indeed Lomas admitted he was worried about his job - unsurprising considering the calls for his head that reverberated around the stadium at full-time.
"Some of the chants heading my way are understandable," said Lomas.
"It's not just in my time, they've had a long, drawn-out misery for the last six, seven months.
"You are always worried about your job. When you are not winning football matches, it goes without saying that you're under pressure.
"I'm not deluded enough to think that's not going to be the case. I'm the manager, the buck stops with me. I'm not shirking my responsibility.
"If I was a fan today paying for that, I wouldn't be happy. The players have to show more drive. When things aren't going right, you've got to stick together and still keep doing the right things.
"It certainly wasn't what I was expecting after the two weeks we've had and the positive performances.
"But it was the same old Achilles' heel, another set piece. Once we conceded the goal, people were nervous and didn't want to get on the ball.
"A few of the lads crumbled and didn't want to take the responsibility of changing things around."
In contrast Derby first-team coach Garner could not have been happier with his team's performance, especially the display of Bryson, who claimed the first match-ball winning display of his career.
Victory in London meant a fourth successive away league win of the new season - their best run since the 1912-13 campaign.
It is form most recently displayed in their last Premier League promotion season.
"We're absolutely delighted," Garner said.
"To come here, get the three points and score five goals - we'd certainly have taken that before the game.
"I'm sure Craig would have mentioned it, if he'd scored a hat-trick before, so I'll take it from that that he hasn't.
"But they were all absolutely superb finishes. We were just delighted he's had a shot. We keep telling him 'when you're 20 yards out, have a go'.
"But we've just told him that you take three goals away and he's still had an absolutely magnificent performance. He's covered every blade of grass. He was setting things up and getting us going.
"Everywhere is a difficult place to go, but this compares well with our other away wins this season, especially considering the goals.
"Millwall are a bit like us at home - having a bad time of it - and obviously the sending off doesn't help. All we can do is prepare right, put a performance on and we've certainly done that."