"I'm absolutely delighted for him that he could get on the end of it. When he came on, I thought he was brilliant and I thought he changed the game," he added.
Liverpool were on the brink of witnessing their first defeat in this season's Premier League.
Manchester United took a 1-0 lead during their match against Liverpool on Sunday.
As the match was inching toward the full-time, Lallana came to the rescue and scored a brilliant goal in the 85th minute to help Liverpool level the scores.
"To get that goal, it will do a world of good for him and his confidence and I think we'll see a lot more from him," said Robertson.
Liverpool still top the Premier League points table with 25 points and will now compete against Tottenham in the league on October 27.