Fans spotted Alisson's reaction to team-mate Caoimhin Kelleher saving Kylian Mbappe's penalty during the Champions League clash between Liverpool and Real Madrid.
After both sides went into the half-time interval locked at 0-0, Alexis Mac Allister gave Liverpool the lead on 52 minutes.
But minutes after the Argentinian saw a shot blocked that could have made it 2-0, Real won a penalty at the other end for a foul on Lucas Vazquez by Andy Robertson.
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VAR agreed with the referee's decision upon review, and Mbappe stepped up to take the spot kick.
The Frenchman attempted to divert his effort into the right corner, but Kelleher pulled off an excellent diving save to deny him.
The Republic of Ireland international has enjoyed an extended run in the team following a hamstring injury suffered by first-choice goalkeeper Alisson.
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And fans spotted the Brazilian's reaction to the penalty save while he was in discussion with one of the Liverpool coaching staff.
The goalkeeper laughed and joked with the staff member before returning his gaze to the action, clearly appearing to be impressed at what he had seen.
Alisson appears likely to return to the Liverpool starting XI once he is passed fully fit, having narrowly missed out on the matchday squad against Real.
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But the 32-year-old might find that path back to the first team slightly more challenging than in the past, given how well Kelleher has played in his absence.
After producing a player of the match performance in Republic of Ireland's 1-0 win over Finland - in which he saved another penalty - during the international break, former Reading and Wolves striker Kevin Doyle tipped Kelleher to become either the 'top one or two goalkeepers in the world'.
He told Irish station RTE: "I think in five or six years he is ... he's that good at everything he does.
"He has got everything in his locker. He ticks every box. If he can get himself playing I feel he's going to be the top man."
Topics: Alisson, Liverpool, Real Madrid, Champions League, Kylian Mbappe, Fan Reactions