The forward was the unlucky one to be hooked by boss Jackie McNamara after Aaron Muirhead's first-half dismissal with captain Alan Archibald being sent on to shore up the back four.
The afternoon went from bad to worse for Lawless as he saw the Pars book their place in the last 16 despite a second-half Thistle onslaught.
McNamara's side return to action with an Irn-Bru First Division clash at Cowdenbeath on Saturday – weather permitting – and Lawless is keen to get down to business.
He said: "Going down to ten men was going to make it difficult, but the boys were brilliant and we were the better team. We had our chances, but it wasn't to be.
"My family were at the game and they were saying that if you came in late you wouldn't have known which team had ten men because we had so much of the ball and a lot of chances. We were unlucky.
"It is always disappointing to be taken off, but it had to be done. It was either going to be Chris Erskine or me. I can't complain, though.
"As soon as I knew someone was going off I put my head down and turned away, but Stuart Bannigan was quick to tell me it was me. I think he was just pleased it wasn't him.
"I thought I started the game quite well, but it is one of those things. I can't wait to play on Saturday now."
Thistle head to Central Park on top of the table and keen to record their third win of the campaign over the Blue Brazil following a Firhill league triumph and victory in the Ramsdens Cup semi-final.
Lawless said: "It is a tough place to go. It was the first time I had played there and it was a difficult pitch. It is not the biggest park to try and play good football on, but we will stil try and do that. If it turns into a battle then, of course, we need to be able to handle it.
"We now know what to expect and I think that will help us a bit. Cowdenbeath have done well this year after coming up, so we will need to be at our best."