The former Gunners midfielder and his team-mates were powerless to resist as their midweek cup exploits at Middlesbrough took their toll in the face of a Thierry Henry-inspired fightback.
Henry's injury-time strike at the Stadium of Light secured a 2-1 victory which lifted the visitors back into fourth place in the table and handed them a psychological advantage for next weekend's repeat fixture in the FA Cup fifth round.
However, Larsson is hoping the Black Cats can make the most of Arsenal's Champions League trip to AC Milan in midweek as they attempt to gain revenge.
He said: "We said before the game, we know it's Arsenal, but we are trying to reach that level where we shouldn't worry about who it is coming here.
"It was a game we shouldn't have lost, but we did, so we need to pick ourselves up and make sure we beat them next week in the FA Cup.
"Hopefully they will be a bit more tired next week than we are.
"We have got a week to prepare now, but at the moment, it is just very disappointing.
"We haven't tasted defeat that many times since the new manager came in, and that makes it even harder to take when it comes around.
"I know it's a team like Arsenal, but we shouldn't have lost that game today [Saturday]."
For all Arsene Wenger's men dominated possession for long periods, Sunderland keeper Simon Mignolet did not have a save of note to make until a dramatic final 15 minutes.
Indeed, the sides were level at the break, but with the Wearsiders firmly believing they should have been in front after the ball struck defender Per Mertesacker's arm as he attempted to control Jack Colback's 39th-minute cross.
Referee Neil Swarbrick allowed play to continue, but Black Cats boss Martin O'Neill was convinced he had got it wrong.
He said: "On most other occasions, I think a referee would have given it. On this occasion, he chose to ignore it."
Sunderland did eventually take a 70th-minute lead in curious circumstances, and again, Mertesacker was heavily involved.
The unfortunate German international caught his studs in the turf under no pressure, damaging ankle ligaments in the process and allowing James McClean to run clear and fire past keeper Wojciech Szczesny.
However, the lead lasted only five minutes as substitute Aaron Ramsey beat Mignolet with a shot which hit both posts, and that proved only a precursor for what was to come.
With the game having entered stoppage time, two more substitutes, Andrey Arshavin and Henry, combined for the latter to stab home from close range to secure victory.
Larsson, who played with the Frenchman during his own spell with the Gunners, admitted there was an inevitability about his intervention.
He said: "We would have liked no-one to have won the game for them, but I suppose if anyone was going to do it, it was going to be him.
"It was his last league game, so it was probably written in the stars.
"From their point of view, fair play to him for getting the goal, but it's obviously a sickener conceding that late.
"You could see in the last 15 minutes that we were really tired and we couldn't really get forward as much as we did earlier.
"But then again, for all the possession they had, I don't think they caused us any problems whatsoever, and they were two really bad goals for us to concede, unfortunately."
The one plus for Sunderland looking ahead to the cup game is that Henry will not figure following the end of his loan spell from the New York Red Bulls.