The substitute suffered a suspected broken leg at McDiarmid Park in a game Saints had looked the more likely to win.
The wheels came off for the hosts on 79 minutes as they endured 11 minutes that were about as bad as it gets for any manager.
It started when Nigel Hasselbaink saw his penalty saved by Ben Williams and Lomas was then sent to the stand by referee Craig Thomson soon afterwards.
Hibs' Paul Cairney added to his woes by netting what proved to be the winner after 82 minutes.
Then midfielder Robertson was taken off on a stretcher and Saints captain Dave Mackay was shown a straight red card for an altercation with Leigh Griffiths in stoppage time as the hosts' misery was complete.
Lomas said: "It's been a disappointing night, especially with Davie Robertson getting that bad injury. Fingers crossed it's not as bad as we think.
"It looked like a block tackle and it looks like he's come off second best, very disappointing for the lad and for us."
Lomas' dismissal from the touchline came after Hasselbaink took the spot-kick following David Wotherspoon's trip on Mackay.
"I kicked a water bottle," Lomas added. "It was frustration. I said to the fourth official he could've used a bit of common-sense.
"It's frustrating, you've missed a penalty, not your regular penalty taker. I was a bit perplexed why he (Hasselbaink) was taking it. Dave Mackay's normally our penalty taker.
Mackay was sent off late on after Griffiths chased down the ball which had been returned to Saints following Robertson's injury.
Lomas added: "Dave suggested that Leigh came and put the head to him. It was a lock of heads and he (Thomson) just chose to send Dave off. If Dave has (motioned his head) it's stupid."