Mitchell was cited for the tackle in which he lifted the Ireland winger up off the ground in the 52nd minute of the Lions' 35-0 victory on Tuesday.
"I found that the incident was clearly at the lowest level for this offence," judicial officer Nigel Hampton said in a statement on Wednesday, banning Mitchell for one game.
Hampton added he had reduced the penalty from its entry level of a four match-ban because of Mitchell's good disciplinary record and the fact he felt a two-match suspension would be disproportionate for the offence.
The Rebels, who are in 12th place in Super Rugby on 32 points and have no chance of making the playoffs, have just one more match this season, against the Otago Highlanders in Melbourne on July 12.
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