Larsson will make his Premiership bow at 35
"We will see how much playing time there will be. Am I a back-up? Yes, at the beginning.
"Then you will see what I can contribute and how things are going. We have not discussed a jersey number at all. We will see what it will be."
Larsson left Barcelona at the end of last season to join hometown club Helsingborg, helping them to a fourth-placed league finish and the Swedish Cup in his first season.
He had spent two years in Spain and played a decisive role in this year's Champions League final when he came on in the second half and set up both goals in the defeat of Arsenal.
The Swede joined Barca in 2004 following a hugely successful seven-year spell at Celtic, where he is widely hailed as one of the best players in the club's history.
He scored 242 goals in more than 300 appearances for the Scottish club.
Former Celtic boss Martin O'Neill was recently asked if he might be interested in signing Larsson for current club Aston Villa, but said he had not been in contact with the striker.
The pair will meet again on 13 January when United meet Villa at Old Trafford.