Alisson's early-season injury could potentially have been catastrophic for Liverpool. The Brazilian won the Premier League Golden Glove last season and helped the Reds to European glory. Those exploits have also seen him nominated in the 30-man shortlist for the Ballon d'Or. Losing a goalkeeper of his calibre would usually be to his side's detriment, but Adrian has come in and filled the void - even if he didn't look capable when his howler allowed Danny Ings to score in the Southampton game.Despite only being at Anfield as a back-up, he's helped the league-leaders to the joint-best defensive record in the top flight.
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They are currently tied with Sheffield United, having conceded just seven goals in their opening nine games.
In fact, his personal numbers are even more flattering.
Two of the goals conceded by Jurgen Klopp's men this season, against Norwich City and Manchester United, were let in by Alisson.
That leaves Adrian with a clean sheet record of 28% and a burgeoning reputation.
It also seems he has adapted to Liverpool's way of playing. Just like Alisson, he is proving an instrumental cog in Liverpool's build-up play, which starts at his feet.
Embarrassingly for Manchester United, although it's not altogether surprising, stats have emerged which show Adrian's build-up play is in fact better than all but two of their players.
The stopper's xGBuildup is 1.80, which is outdone only by Paul Pogba and Scott McTominay.
It's also better than the entire Everton squad.
The metric xGBuildup is a great way of recognising unsung heroes. It measures players' involvement in build-up play every time they get possession, but removing shots and assists.
That allows for more effective analysis of players further down the pitch, not simply those who are more likely to be directly involved in goals.
Just take a look at Fabinho, too - although it's pretty alarming reading for Jordan Henderson:
Gone are the days when Klopp was left scratching his head over Loris Karius and Simon Mignolet.
It's unlikely we'll be seeing much more of Adrian for the meantime, however, now that Alisson has returned to fitness.
Who'd be a back-up keeper?