Having hit back from 16-0 down to trail only 17-14, Leeds had a chance to go ahead when Jack Walker made a break, but was tackled high by Catalans Dragons winger Jodie Broughton.
The Leeds full-back played on, but the opportunity had gone. Referee James Child awarded a penalty but allowed Broughton to stay on the field and Catalans went on to win 33-20.
“If (Walker) stays down and the bloke who made the high tackle gets sin-binned, we go on and win the game, but I tell my players to get up,” said McDermott.
“When Greg Bird stayed down to try and get everybody’s attention to what wasn’t a crusher tackle at all, when Ashton Golding was on him, those players are harming the game of rugby league.
“That’s a soft thing to do. I will always tell my players to get up. I don’t think the game was lost there and then when Jack Walker got up and tried to carry on running but it’s one of those moments that you feel hard done by.”
That said, McDermott admitted Leeds’s poor start caused their downfall.
Benjamin Jullien, Benjamin Garcia and Michael McIlorum scored early tries for the hosts.
Ash Handley touched down after Sam Moa was sin-binned for a shoulder charge on Richie Myler, then – after a drop goal from Tony Gigot – Brett Ferres and Ryan Hall crossed.
But tries by Gigot, Josh Drinkwater, who also kicked four goals, and Broughton made the game safe for the hosts before Brad Dwyer grabbed a late consolation and Jordan Lilley kicked his second conversion for the Rhinos.
“There’s more in us than we showed, we’ve got more resistance in us,” insisted McDermott of his injury-hit side.
“They should be very disappointed with the start. When they started to use their shoulder and started to bang a little bit, it became a contest.”