Joshua is expected to defend his IBF, WBA and WBO heavyweight titles against mandatory challenger Pulev in June with the Tottenham Hotspur Stadium in pole position to be the host venue, Sky Sports reported.
"Because of the Andy Ruiz Jr. fight there is an element of 'he is beatable'," Joshua's promoter Hearn told Sky Sports.
"We've seen Joshua get beaten – just land one on the chin, or the side of the head, and anything can happen? Easier said than done."
Joshua told Sky Sports this week, "I don't want to do what Andy Ruiz Jr. did, and lose it straight away. I want to hold it for a number of years.
"Trust me, a different beast has been awoken."
Pulev, 38, from Bulgaria has lost just once in 29 fights, in his sole world title fight against Wladimir Klitschko in 2013. He has since beaten Derek Chisora and Hughie Fury.
"There is always fear in the heavyweight division," Hearn said.
"They are bang up for it.
"One thing that Pulev has got is great activity. He's getting older but he has been nice and active.
"Ivaylo Gotzev is Pulev's long-term advisor, friend and hometown promoter – deluded is the wrong word but he is so close to Pulev that he believes this is their moment, their time. The stuff they saw in the Andy Ruiz Jr. fight? They think they can beat Joshua.
"Then there's [his promoter] Bob Arum who is telling everyone that Pulev is going to knock Joshua out. Maybe that's just Bob Arum – 'yesterday I was lying, today I'm telling the truth' is his famous line.
"I feel that Joshua vs. Pulev is a really good fight. Two big standup heavyweights who can really punch. Pulev has a lot of experience and good ring IQ.
"Joshua will look to make a statement. He had to box that way against Ruiz Jr. because there was no room for error but now people are saying, 'Show us the knockouts again'.
"The pressure is on AJ to produce a knockout. Two big men will trade shots."
Pulev edged Oleksandr Usyk, the WBO mandatory challenger, in the race to fight Joshua first. Usyk is expected to face Chisora next instead but later than planned.
Hearn said about Usyk, "He started camp for March 28 but his elbow was still a bit sore from his last fight. His physios and doctors said he was better off delaying it by four weeks.
"They wanted to fight at the end of April but we couldn't get a venue so we move Usyk vs. Chisora to the end of May. They didn't want to arrange a fight without being 100 per cent. It will take place this summer, virtually guaranteed, we are just finalising the venue.
"Same as Dillian Whyte vs Alexander Povetkin – done deal."