Already having to contend with the unexpected departure of talisman Lionel Messi, Barca are in a dire financial situation heading into the new season.
They will now also be without one of their new arrivals for up to 10 weeks, though LaLiga's salary restrictions have meant Barca have not yet been able to register Aguero, who joined on a free transfer from Premier League champions Manchester City.
The 33-year-old left City after 10 years with the club, but his final season was hampered by injury and COVID-19 lay offs.
He managed only eight starts in 2020-21, playing 720 minutes in total across 20 appearances in all competitions.
Aguero's tally of six goals – two of which came from the penalty spot – was by far the fewest he managed for City since joining in 2011. Indeed, his previous low was 17, in the 2012-13 season.