Saracens boss Mark McCall slammed the touch judge involved in the sending-off of flanker Justin Melck and vowed he would try and get the "dreadful" decision overturned.
Melck was red-carded four minutes into the second half of Saracens' 31-27 Aviva Premiership defeat away to Leicester for appearing to eye-gouge hooker Neil Briggs, who later said the officials had made a mistake.
Referee Greg Garner sent off Melck on the advice of touch judge Simon McConnell and did not ask the video referee to check the decision.
McCall said: "We are hugely disappointed with the decision.
"It seems very unusual for a touch judge, who is in the position he was in, to make that kind of call and decide not to consult the fourth official.
"Their player did not complain and subsequently said it did not happen. To make a call of that magnitude with 40 minutes left is staggering. We will look to have the decision over-turned. Justin is not that kind of player, it was a dreadful decision."
McCall was backed by Leicester's director of rugby Richard Cockerill, who said: "Neil Briggs was the supposed victim and he said there was no eye-gouging.
"It was just a hand across the bottom of his face. He said it was a mistake by the officials.
"I don't think Melck is a dirty player and I don't think he did it. Hopefully, it will get resolved by the authorities."
Melck, who could be banned if the decision is upheld, was sent off in the 44th minute, soon after winger Michael Tagicakibau had been sin-binned for a tip tackle on Marcos Ayerza.
The decisions proved a turning point in the game for Leicester who clinched third place with victory.
They will meet great rivals Northampton away in the semi-finals next Friday, while Saracens will be at home to Harlequins, who sealed fourth place by beating Bath.
The Tigers, who led 10-8 at half-time, took advantage of Saracens' woes to surge into a 24-8 lead but could never shake them off and in the end were relieved to hear the final whistle.
McCall praised a side shorn of most of their top 30 players who had been rested with the Heineken Cup final and, possibly, the Premiership final on the horizon.
He said: "If our very senior team had responded in that way I'd be pretty pleased. To have the team that was out there, with a number of 18,19 year olds in it and people playing for Saracens for the first time... I thought they were magnificent."
McCall said the hurt of losing last year's semi-final to Northampton would be a huge spur next weekend, adding: "We were disappointed with how we played in last season's semi-final.
"That disappointment has been there for 12 months, and all our focus is going to be on our preparation for Quins. Whatever happens the week after (Heineken Cup final v Toulon) is irrelevant. We are desperate to get through to the final."
Leicester scored four tries through Manu Tuilagi, Blaine Scully, Graham Kitchener and a penalty try. Saracens also scored four, with Ben Spencer getting two, the others from Jack Wilson and a penalty try.
Cockerill said three of his injury victims from the match - Boris Stankovich (bang on head), Owen Williams (quad) and David Mele (bang on head) would be fit for next week, while Geoff Parling (bang on head) would be assessed in the next 48 hours.
He praised Saracens for their performance and said that having got to the semi-finals they wanted to hold on to the trophy they won last year by beating Northampton.
He said: "Saracens and Northampton have been the best sides all year.
"But we are a good side when we get it right. We have done well against Northampton in the last few years, it will be a hell of a game, local derby, semi-final... all the pressure is on them.
"They have had the season of their lives and they have got to finish it off. If you get to this stage you have worked hard, so you want to try and win it.
"Play-offs are brutal. You can be top and lose a semi-final. It's heartbreaking and we have all been there. We want to try and win the thing."