Liverpool went 1-0 down after just one minute as Danny Ings flicked over Alisson Becker from a neatly-worked Southampton free-kick and never recovered, leaving them level on points with United having played a game more.
Jurgen Klopp was forced to name midfield duo Jordan Henderson and Fabinho as his two centre-backs due to Virgil van Dijk, Joe Gomez and Joel Matip all being sidelined through injury.
And with Liverpool unlikely to add any new centre-backs during January, it seems Klopp will have to patch together his side from now until the end of the season.
The Reds have endured a tricky few games over the festive period, having dropped points against Newcastle and West Brom prior to the trip to St Mary’s.
But Robertson believes there’s still a road to the title for Liverpool, and explained he believes the title will be won with a points tally in the 80s rather than the high 90s – as in previous years.
Liverpool secured the title last year with 99 points, having come second with 97 the previous year after Manchester City clocked 98.
Speaking before the trip to the south coast to face Southampton, Robertson said: “Over the last two years or even three years – and no disrespect to those clubs – but last season we would have won those games.
“If you add two points to every one of those games then the league table is looking a lot different and we’re sitting pretty happy, but you know, this season is going to be unique.
“I think there are seven or eight teams – once the games in hand have been played that are within three points of us at the top, and it’s going to be a unique season as I said.
“I think I can comfortably say it’s not going to be 97 or 98 points that is going to win the league this season, it’s probably going to be in the 80s because it is that tight and everyone is taking points off each other.
“We need to stay in the race, and that’s what we have done so far.”