Mac Allister's superb second-half strike secured a debut 2-1 UCL win for new Reds boss Andoni Iraola and got the Argentinian off the mark for 2026/27.
His impact against Atleti comes days after a public statement of 'sadness' and disappointment over the club lack of movement on a new deal for the midfielder.
The former Brighton & Hove Albion playmaker is still on the same contract he signed upon his arrival at Anfield back in 2023, which expires in 2028.
Fellow midfielders Ryan Gravenberch and Dominik Szoboszlai were both given extended deals earlier this year, but Mac Allister has been left in the dark over his position, which hints at the club being unsure over his place in their future plans.
Mac Allister's openness on the situation has split fan opinion and club icon Steven Gerrard was clear in his message to the 27-year-old.
"Let your football do the talking," Gerrard told TNT Sports.
"Consistency, top performances, show your experience. Show you're a top international. The club will reward you."