Sporting head into the first leg of their Europa League quarter-final against Metalist Kharkiv on Thursday.
The Portuguese club beat City on the away goals rule after their tie finished 3-3 on aggregate to progress, and Xandao told sporting.pt he is expecting another difficult challenge against the Ukrainian side.
Sporting are unbeaten in their last 10 European matches on home soil, but Xandao said: "It will be a tough game, but it is an important one if we are to achieve our objectives in this competition.
"We are feeling confident right now, especially after beating Manchester City.
"But we can't let us knocking Manchester City out of the competition go to our heads or start thinking that it will be an easy game.
"Like Manchester United, they were considered as one of the competition's probable finalists. However, we can't let our focus slip now and Metalist is in the quarter-finals for a reason."
The 24-year-old Brazilian, who scored the winner against City in the first leg at Estadio Jose Alvalade, is hoping home advantage could be important in deciding the outcome of the tie against a Metalist side who are the competition's top scorers with 25 goals.
"We have to make use of the game in Alvalade to get ahead in the tie, because in the Ukraine Metalist are really strong," he said.
Metalist boss Myron Markevych, whose side saw off Greek giants Olympiacos to reach this stage, also has Sporting's upset over City at the forefront of his mind as he prepares his side for the match.
"I watched their last game in the Europa League against Manchester City," he said.
"The team played at a good level and knocked out one of the favourites for the competition - one of the best teams in Europe.
"It only indicates the potential of Sporting."