However, Fuller has been told that the knee does not require surgery and after observing Fuller's early season form and have signed the 25-year-old on a pay-as-you-play basis.
Chief Executive Peter Storrie released a statement on Portsmouth's official website stating:
"Originally as a club we just could not take the gamble on paying out a lot of money given the advice we were given.
"We then watched him against Watford and have now concluded a deal that is sensible for all concerned, giving Ricardo his deserved chance to fulfil his Premiership ambitions and at the same time insuring us against further damage to his knee".
Fuller signs in time for the trip to Charlton, but is expected not to feature as Portsmouth hope to claim their first away win of the season at the first time of asking.