Leeds coach Brian McDermott questioned the dismissal of centre Joel Moon which proved decisive in his side's 13-12 defeat by St Helens.
The Challenge Cup winners were forced to play for an hour with 12 men after Moon was shown a red card by referee Richard Silverwood for an illegal shoulder charge on St Helens centre Mark Percival, who was taken off with concussion and took no further part in the match.
The Rhinos went on to build a 12-2 half-time lead with a try from Liam Sutcliffe and four goals from skipper Kevin Sinfield, but their defensive efforts eventually took their toll.
"I only saw it a couple of times in the throes of the game so I'm not 100 percent sure, but it didn't look like he made contact with the head and, if he hasn't, then it's a big call, a wrong call in my opinion," McDermott said.
"I thought it was ultimately the difference between the two teams.
"We used up that much juice defending. It has to be said we made a couple of errors, but how much can you ask of a group of 12 men in a game as intense as that?
"Credit to St Helens, though, they got the job done."
Saints turned the game on its head with two tries in seven minutes through Louie McCarthy-Scarsbrook and James Roby to level the scores and stand-off Lance Hohaia secured the win with a drop goal seven minutes from the end.
With second-placed Wigan suffering a shock defeat at Widnes, the win left St Helens five points clear with two games left of the regular season and now only Castleford, currently in sixth place, can prevent Nathan Brown's men from lifting the league leaders' shield.
Brown said: "Leeds are never dead, they fought hard with 12 men and got away to a nice lead on the back of a great kicking game and great control from Kevin Sinfield. I'm really proud that we were able to hang in there.
"We had a lot of defensive work to do and I thought our edges did a super job in handling (Ryan) Hall, (Zak) Hardaker and (Kallum) Watkins.
"I know people will talk about the 12 men, but I thought Leeds were just sensational in that first half. They put us under a lot of pressure and we probably folded a little bit.
"In the second half we started to get going a little bit and hopefully it will be a learning curve for us. I am really proud of the guys.
"Obviously we'd love to finish first, but regardless of where we finish, after what we've been through this year, I don't think any of the fans could be disappointed with what the players have given them. I'm really proud of the guys regardless of where we finish."
Brown said Percival would be forced to sit out next Thursday's game against Warrington at Langtree Park, where Saints can clinch the shield, but he had some sympathy for Moon.
"I didn't get a real good look at it so I can't really comment on it and I don't want to take about it because Leeds have just had a sensational Challenge Cup victory and I really think their players are great ambassadors for the game," Brown said.
"Joel Moon is not a dirty player and Percy is going to be okay, that's the key. He might not be able to play next week, but that's only because of the concussion rule. Joel Moon will be disappointed enough and, if he got off with a warning, I wouldn't be disappointed."