Dan Tuohy has avoided being cited for the high tackle that hospitalised England fly-half Toby Flood.
Flood, the Leicester skipper, was knocked out during last Saturday's pre-season friendly against Ulster at Welford Road and spent the night at Leicester Royal Infirmary where scans revealed no further damage.
The Ulster lock was sin-binned for the offence but the RFU have revealed they are unable to take any further action due to a disciplinary loophole.
However, the injured 28-year-old Tiger will miss this weekend's Premiership opener against Worcester.
An RFU spokesman said: "Ulster players were not subject to the RFU's jurisdiction in this match and, as a result, no action can be taken.
"Clubs can agree how discipline should be dealt with in advance of such games and we're looking to see how we can make sure this always happens in the future."
The inability to cite Tuohy will further infuriate Leicester director of rugby Richard Cockerill who earlier this week called for the incident to be investigated.
Cockerill said last week: "Once again, our playmaker has been hit and we have lost him from the game. It is illegal. You would expect the powers-that-be to have a look at that."