Liverpool were beaten in the FA Cup at United, before a 2-2 draw in the league saw their title hopes collapse.
Robertson said: "Getting a point at Old Trafford can never be a negative, in the past or in the future, when it's one of the oldest rivalries. When you go to these places, of course you want to win.
"But if you do walk off with a draw you can't be overly disappointed. On that day, I think that if we were a bit more clinical we would have got the three points - but it was other results that cost us in the end.
"Crystal Palace at home, West Ham and Everton away. That week where we didn't pick up enough points was when we realised we weren't in the title race anymore. I really don't think the United game was a moment that made us think we were out of it.
"Obviously we've moved on from that but last season did hurt there. The 4-3 game, we dominated for a large part and played good enough to win the game. But sometimes these things go against you and that is what happened that day and it was unfortunate."