The former Huddersfield Giants second-row is one of three Rhinos players who miss tomorrow’s game against Leigh Centurions following Sunday’s feisty win at Salford Red Devils.
Mellor appealed the one match penalty notice but yesterday saw the suspension upheld by the RFL’s disciplinary panel.
England scrum-half Luke Gale was banned for two games for punching and New Zealander Bodene Thompson was hit with a one-match ban for a similar offence.
Rhinos hooker Brad Dwyer (dangerous contact) and winger Tom Briscoe (contact with the match official) were also charged but not suspended.
Salford’s Ryan Lannon avoided a ban for punching Gale and Lee Mossop was suspended for two matches after a punch that floored Konrad Hurrell, the Leeds centre who now misses two games due to concussion.
Salford’s Jack Wells (dangerous throw) also got two games and Joe Burgess received a one-game ban for punching.
Agar said: “It was a costly two points.
“I really don’t think we’re a poorly disciplined team.
“Konrad Hurrell misses two games with a head injury and we had two blokes - one suspended, one not - on charges of touching the referee.
“I’m not sure if it’d knocked the dandruff off his collar but they are applying the laws.
“The guys at match review and the panel last night, we had a really fair hearing with the judge and two ex-Leigh players on the panel, and they have to stick to the rules.
“Unfortunately we lose Alex Mellor in a really, really tough period of fixtures.
“They’re sticking to the rules but it’s a careless incident at best and we lose more players.”
In mitigation for captain Gale, Agar added: “Luke (Gale) threw punches. “He had a knee placed on his neck which is very, very dangerous.
“Unfortunately the cameras didn’t pick that up.
“Bodene (Thompson) threw a punch (at Mossop after Hurrell incident) but last year we played Catalans and Brad Dwyer got smacked in the mouth.
“I thought we had a poor reaction from our players to that and it didn’t incur a penalty.
“It did subsequently at match review but not on the night and I think our players certainly made a pact to each other they wouldn’t allow that to happen again.
“When one of your players gets knocked out, there’s no side in the competition where you wouldn't see a reaction from their team-mates.”
Winger Ash Handley is also absent tomorrow due to a head injury picked up while on his England debut.
Leeds also visit Warrington Wolves on Monday and Agar added: “It’s disappointing the head injury situation with Ash Handley and Konrard Hurrell.
“And the fact we have games back to back doesn’t help - the schedule.
“But we’ll roll our sleeves up and get on with it and see what this week and next throws at us.
“There’s not been many weeks gone by when we’ve not had to deal with some issue of sorts. And a lot of them are not issues of our own doing really.”
Agar does have stand-off Robert Lui set to play his first game of the campaign having suffered a quad muscle injury in pre-season while fellow half-back Callum McLelland is also back to play having not featured yet in 2021 due to ankle and groin problems.
Youngsters Liam Tindall and Levi Edwards are also in Leeds’ squad.
Tindall, a 19-year-old winger, made his senior debut last year, but 17-year-old centre or second-rower Edwards has yet to play in the first team.
Full-back Richie Myler suffered a dislocated finger against Salford and could be out for three to four weeks but Agar hopes that will be shaved to two,