Aspiration with a needle is necessary to allow the tissue layers to once again "touch" and heal this cavity shut. If a seroma is allowed to persist, a pseudobursa forms, and the longer it remains undrained, the less likely the "slick" surfaces of the pseudobursa will adhere and heal. So waiting is bad. If repeated aspiration is not successful in "encouraging" your body to close off this cavity, then a drain may need to be reinserted (local anesthesia--not a big procedure at all) so that sclerotherapy can be performed via the drain.
Sclerotherapy involves placing a solution via the drain into the cavity to cause irritation and inflammation of the pseudobursa lining--in essence, making it "sticky"--so that tissue adherence and healing is stimulated. Sclerotherapy can also be repeated several times and this almost always works at sealing off these seroma cavities. Tetracycline (the antibiotic, but used as a solution, not as an oral pill) is a common sclerosant used by surgeons of several specialties.
If sclerotherapy doesn't do the job, then re-operation may be necessary. That is both a big deal, and a costly one (especially for a high location seroma), so tending to ANY seroma promptly and repeatedly (aspiration) or promptly and continuously (drain reinsertion) is really in your best interests. Most patients don't want a drain reinserted, and most surgeons would rather not have to do even a small procedure if not absolutely necessary, but if aspiration doesn't seem to rapidly decrease the amount of fluid at each session (best done every few days, not every week or two), I'd recommend a drain so that continuous removal of the fluid keeps the tissue layers in contact with one another for healing (and allowing sclerotherapy to "jump-start" the process). Good luck; now go call your surgeon for an immediate appointment, not next week!