Zarate was signed for West Ham during the 2014 season, but he has only just started to show his true quality.
Sam Allardyce was loathed to use the Argentine during his time as West Ham boss, but Slaven Bilic has restored him to the side’s first team and the Croatian has been rewarded for his faith.
Zarate has scored five goals in 15 appearances so far this season, and Sullivan was very impressed by the Argentine’s free-kick against West Bromwich Albion in the Hammers’ latest outing.
'It was a great free-kick from Mauro Zarate to put us into the lead," Sullivan said. "When I bought Zarate I knew he was and is a top player and he is very important to us."
Zarate’s free-kick provided West Ham with a lead to protect against the Baggies, but they were unable to hold on and claim victory.
West Brom ended up drawing 1-1 with Bilic’s side, but Zarate will still be pleased to have made an impression in the fixture.
West Ham need Zarate to step-up, as they are in need of injecting some creativity into their side, following an injury to Dimitri Payet.
Payet is set to be out for the next three months, and Zarate looks the most likely replacement for the Frenchman in Bilic’s squad.