The teenage son of former Reds and Wallabies star Michael has burst onto the scene this season, forcing his way into new Australia coach Eddie Jones' Test calculations with his poise and kicking accuracy in the competition's opening month.
But the 19-year-old has been scrubbed out of Saturday's clash in Melbourne against the Rebels and will likely miss next Friday's home game against the Crusaders as he follows World Rugby's 12-day return-to-play concussion protocols.
A late substitution in the Reds' tight defeat of the Fijian Drua on Sunday, footage shows Lynagh attempting to tackle try-scorer Iosefo Masi.
The play is clearly dead when Ratu Rotuisolia joins the pile-on and collects Lynagh from behind.
Queensland forward Sef Fa'agase was suspended for three weeks after his tackle on Eroni Sau in the same match concussed the Drua winger.
The hit went unnoticed on the field but was raised by Drua coach Mick Byrne in the post-game press conference.
Fa'agase was retrospectively cited and issued with a red card, with a six-week suspension downgraded to three thanks to his clean record and early guilty plea.
AAP understands the Reds presented Lynagh's incident, as well as a potential high tackle on winger Filipo Daugunu earlier in the game, but no action was taken.
The ban stretches the Reds' already-thin forward pack - Taniela Tupou and Harry Hoopert both have long-term injuries - and leaves Lynagh, who has recovered well, a frustrated bystander.
"He's had a tremendous start to his career," Reds co-captain Tate McDermott told AAP.
"I have been surprised, but I knew what he was capable of.
"He'd be bloody pleased with how he's gone, I know we are. But he's a small frame, so give him some time to get back on his feet and recharge."
Reds winger Suliasi Vunivalu will also miss the Rebels clash after he cramped up late against the Drua and failed to finish the game.
Vunivalu has battled hamstring injuries since switching codes, the Reds opting for caution as the former Melbourne Storm NRL winger rediscovers top gear.
Jordan Petaia is set to return after he missed the Drua clash, while Lawson Creighton or Isaac Henry, who has recovered quickly from surgery to fix a hand broken in round one, could replace Lynagh.
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