The club-record signing has struggled to make an impact at Vicarage Road this season since his move from Granada and again could not make the starting line-up on Saturday.
He was introduced as a late substitute for Tom Cleverly with 10 minutes to go at Selhurst Park but failed to salvage anything for the Hornets, who went down 1-0 to Crystal Palace.
“We have some players like Nordin Amrabat who need to find their fitness,” Mazzarri told the Watford Observer.
“Isaac Success is the same. As well, now we have (M’Baye) Niang who also plays in the same role, and (Stefano) Okaka and (Troy) Deeney, and with Amrabat who can play as winger, and also Daryl Janmaat.”
Mazzarri will be hoping Success improves his fitness and finds his scoring form during the international break when the Super Eagles play two international friendly games against Senegal and Burkina Faso in London on March 23 and 27 respectively.