Passions still run high among some of the Emirates Stadium faithful by the manner in which the Holland forward left last summer in a £24million move, and there have been suggestions those disgruntled fans will turn their backs in protest when the teams come out from the tunnel ahead of a guard of honour to mark United's achievements.
But Spain full-back Monreal said: "Teams like Manchester United don't just have gifted wingers, they have a squad with a lot of depth. Their players are of the highest level, you can't just focus on one player because their team is full of players who can create and score."
Full-back Monreal, who joined Arsenal from Malaga in January, added: "We must be on our guard and be aware of certain key players, like Robin, who has scored a sack of goals this year and is playing well. He is one of their main threats.
"They are one of the biggest teams in England and they are having an exceptional season. It is one of those games every player wants to be involved in, but there are three points at stake and that is what we are aiming for. They may be a very strong opponent, but we are also a very good team and can win the match."
Gunners boss Arsene Wenger, meanwhile, admits losing striker Olivier Giroud to a three-game ban is a "blow" - but one which his side must learn to deal with if they are to hold on to a place in the top four.
Giroud was shown a straight red card by referee Andre Marriner in the final minute of last Saturday's 1-0 win at Fulham for jumping into a challenge with defender Stanislav Manolev. Although Wenger initially indicated he felt the dismissal had been warranted, after review the club appealed, which was on Tuesday rejected by the Football Association.
Wenger must now re-evaluate his attacking options for crucial games against United on Sunday, then at QPR and home to Wigan before Giroud, who has netted 17 goals, will be eligible again on the final day at Newcastle.
"It is a blow that we cannot have him available. We will have to do without him and we will do it," Wenger said.
"It is a harsh decision. After the game, I was a little bit less convinced that he didn't deserve the red card, but having seen it again, I feel it is completely accidental and a very minor incident."That is why I decided to appeal. I am sad that it didn't work, but we have to cope with it."