Success, earlier in July joined Watford from La Liga side, Granada on a club record fee of £12.5m.
As a non-EU player, the 20-year-old has not met the requirement to go straight into Watford team and requires a work permit.
The former Golden Eaglets on Monday, July 11 filed an application to the Home Office of the United Kingdom which was granted on Wednesday, July 13.
He didn't pick up the documents until Tuesday, July 26 and he's now eligible to play for Watford.
Isaac has been involved in Watford’s friendly games so far and reveals that he has been working hard with Watford Coach Walter Mazzarri ahead of the Premier League season.
“I’ve worked with him [Mazzarri] a lot already during my first two weeks and he’s very good, both as a coach and a person,” the 20-year-old told Watford’s official website.
“You can tell that from the work that we’ve done on the training pitch. He knows what we need to do to be successful, and he’s shown that at the clubs he’s worked with before Watford.”