The Nigeria international was one of the stars of last season for the Hornets, scoring 17 goals in all competitions before subsequently being linked with a summer move away from Vicarage Road.
Manchester United and Chelsea were reported to be interested, while Chinese club Shanghai SIPG are believed to have offered £37.5million for the striker.
Watford eventually managed to retain the 27-year-old's services but may be wishing they had cashed in, Ighalo having scored just twice all season.
Ighalo's failure to find the back of the net, coupled with a change in formation, has resulted in him being dropped to the bench by Mazzarri for the last two matches against Stoke and Leicester, leading to speculation the club could look to sell in January.
Mazzarri insists he does not want Ighalo to leave though, claiming his still needs him despite his diminished return.
"I speak about what I know and I speak about that I have seen so far this season," the manager told reporters. "He is a player that I like and I trust very much.
"When he was in a good condition at the beginning of the year he played, and started the games. Of course he is a player that we need.
"He is in competition at the same level with all the other strikers, and if we want to grow as a club and in the future go further in competitions, then you need to have at least six important strikers and Ighalo is one of them."