The Lions, who had only taken one point from their opening four games, looked on course when midfielder Martyn Woolford drove them ahead shortly after the break at Brighton's Amex Stadium.
But Dunne, booked earlier for time wasting, received a second yellow card for a foul on substitute Andrea Orlandi before Argentinian striker Leonardo Ulloa rescued a point for Brighton a minute from time.
Dunne's dismissal was hard for Lomas to take, especially as he predicted it could be coming at half time given what the nature of the first yellow card.
Lomas said: "I said to the fourth official that it was a very cheap booking and I could see what could happen in the second half.
"The sending off was definitely the key part of the game and the turning point as we were comfortable.
"We set up to nullify Brighton who we gave due respect to after they won their previous two games.
"We deserved our goal and to come away with a draw is disappointing really because the players were superb."
With Monday night's transfer deadline looming, Lomas stressed the need to "inject fresh blood into the squad."
He said: "I believe that if we are to really, truly take this great club forward we need some fresh blood and there needs to be an injection of new faces.
"Too many players have become stale. Hopefully we can shift a few on and be creative."
Brighton head coach Oscar Garcia admitted his men paid the price for taking too long to get going.
The Spaniard was not happy with the Seagulls' first-half display and said: "In this league if you do this then it's difficult to win.
"It's not easy to win consecutive games at home, or consecutive matches, but in the second half we played better than Millwall and deserved to score."
Brighton have seven points from five games going into the international break and Garcia believes steady progress is being made.
He added: "If you don't win at home you are not happy, but we are improving.
"It will take time because the players are understanding the changes we are making.
"Now we have a period of time when we can hopefully get players fitter."
One Seagulls player struggling with fitness has been Kazenga LuaLua, but Garcia was encouraged with the way the former Newcastle winger responded after coming off the bench shortly after the break.
Said the Spaniard: "LuaLua has not been 100 per cent fit because of his knee injury and that's why he hasn't played as much as he would have liked.
"What we have now is a chance to get players fit. Then we will pick the best 11."