Nigerian forward Taiwo Awoniyi was hospitalised with a severe concussion after a nasty clash of heads during Mainz's 1-0 loss to Augsburg in Sunday's Bundesliga relegation battle.
Liverpool loanee Awoniyi, 22, was left briefly motionless after landing awkwardly following an aerial collision with Augsburg defender Felix Uduokhai in the first half.
Awoniyi was stretchered off with his neck in a brace after a long delay while he received medical attention on the pitch.
Mainz said Awoniyi, a former Under-17 World Cup winner, was responsive and taken to hospital for observation.
"We have received the all-clear from the doctors," said Mainz coach Achim Beierlorzer.
"He has regained consciousness and can remember the things that happened.
"He has a severe concussion and has to stay in hospital for observation."
Awoniyi has yet to make an appearance for Liverpool since joining the club in 2015, spending numerous spells on loan in Germany, the Netherlands and Belgium.
Augsburg moved seven points clear of the relegation play-off place with just three games to play after striker Florian Niederlechner scored the only goal just 43 seconds into Sunday's match.
Niederlechner was happy to atone for his penalty miss in last weekend's 1-1 draw with Cologne.
"Last week I was the idiot, this week I am the hero," he said. "That was a big step (towards safety), but we can't afford to relax."
Augsburg must still face Hoffenheim, third-from-bottom Duesseldorf and Champions League quarter-finalists RB Leipzig.
Mainz stayed three points above the bottom three.
They came closest to equalising when French striker Jean-Philippe Mateta rounded the Augsburg goalkeeper but had his shot cleared off the line by defender Philipp Max.
Mainz face second-placed Borussia Dortmund on Wednesday before Saturday's crucial home match against Werder Bremen, who are second from bottom and three points behind them in the table.
Mainz forward Taiwo Awoniyi is stretchered off after suffering a serious head injury in Sunday's relegation battle against Augsburg