The Brazilian goalkeeper let a hapless United back into Sunday's meeting between the teams at Anfield as he spilled Romelu Lukaku's left-wing cross into the path of Jesse Lingard at the Kop end.
Fortunately for the man between the sticks, his mistake was ultimately rendered irrelevant as Xherdan Shaqiri's late brace handed the hosts a 3-1 victory.
But Robertson believes Alisson would not have deserved harsh criticism even had it ended all-square after he kept the Reds in Europe's premier cup competition with a last-gasp save against Napoli on Tuesday.
"He has been unbelievable this season and we all make mistakes," the Scot said.
"It is just if Mo Salah or Firmino make a mistake it maybe doesn’t get punished because it is up the other end, but that is unfortunately what a keeper has to put up with, if they make a mistake it usually turns into a goal.
"Of course, he will be disappointed with it but he bounced back and we can’t moan about it, he has dug us out Tuesday night and we are still in the Champions League because of him.
"We have not dug him out of a hole but we have put in a performance and his error hasn’t been highlighted because we have won the game and that is what a good squad does."
In downing the Red Devils following a 69th-minute introduction in place of Naby Keita, Shaqiri continued his recent habit of making important contributions from the bench.
The Swiss arrived at Anfield from recently relegated Stoke City this summer and Robertson, who followed the same route from Hull City 12 months earlier, believes that may have eased the pressure on his teammate.
He added: "He is seizing his chance and good on him. You can use it to your advantage because you might go under the radar a wee bit and there might not be as big a deal made about your signing.
"It [gives you] time to get your feet under the table and Shaq has hit the ground running - quicker than I did - so credit to him.
"He has learned the way to play and he has been fantastic all season and a real threat for us and his stats this season are very good, so long may that continue.
"You look at the bench and there is probably a few people disappointed not to start, rightly so and we have all got positions to fight for and that is the beauty of having a good squad.
"Shaq comes on and shows everyone what he can do and scores the two goals and he will take the headlines but the other subs that came on...
"Hendo came on and shored up the midfield, won every tackle, I think, and never lost a header against Fellaini, which is always tough.
"So these guys were disappointed not to play before kick-off but they come on and make such a difference for us. It was a real all-round performance."
Robertson was singled out for praise by Jose Mourinho following the match, the United boss saying: "I am still tired just from looking at Andy Robertson! Absolutely incredible - he makes a 100m sprint per minute."
The Scotland captain offered a typically humble response to those comments, preferring instead to praise fellow full-back Nathaniel Clyne.
He said: "It is just part and parcel of my game, I will keep on going until the end.
"I got a bit of space, particularly second half and the gaffer is always banging on to me and Trent, more so this season, that we have to be more involved and I was just tried to do that.
"Special mention goes to Clyney today, being out for so long and my God, he done brilliant. He has had his struggles with injury but he put in a first-class performance and that just sums up our squad at the minute."