Slaven Bilic has helped West Ham United challenge for a Champions League place this season.
West Ham’s board have attempted to fine some of the first-team squad for showing indiscipline this season, but Bilic has stuck by his players.
Sullivan has joked the the Croatian boss’s stance has cost him money this season, but he has otherwise been very impressed by the 47-year-old.
“He was a gamble — I told him that,” Sullivan said. “And now I’ve told him he is looking less of a gamble! He impresses me, he is passionate and he talks with sense and intelligence.
“In my 25 years in football I have listened to some of my managers speak after a game and thought: ‘Who is he kidding? Does he think we are stupid?’ But Slaven calls it and I like the way he calls it.
“If we’ve been lucky, Slaven will say that. If we’ve played badly, he will say that. The fans have got someone who feels the game as they do.
“And he fights like mad for his players. When they had fines owed to the club, he came and fought for them and had the fines removed. I’m not sure I liked that... it cost us a few quid! But he fought for them. Fines for indiscipline, that sort of thing.
“He has a gravitas, a strength. I am proud of him and respect him. Now can he do it again next season?”
Bilic has enjoyed a remarkable first season at Upton Park, after taking over last summer.
The Croatian boss was tasked with ensuring that West Ham remained a Premier League side as they prepared for their move into the Olympic Stadium, but he has far surpassed expectations.
Bilic has guided the Hammers to a brilliant campaign, with the London side currently challenging for a top four spot.
West Ham are sat in fifth place in the Premier League table, just one point behind Manchester City, who are in fourth.
The Hammers have also made it through to the quarter-finals of the FA Cup, with Manchester United standing in their way of booking a place at Wembley.