Pacquiao had a number of days off training leading into what he calls the "serious" preparation.
Roach, who has steered Pacquiao to his most impressive victories, said the champ was "way behind" in his preparation and it showed in his sluggishness in sparring.
He said Pacquiao hitting peak form was "not gonna happen overnight. It's gonna be a process."
Pacquiao, who will defend his WBO welterweight title (66kg) against the Fighting Schoolteacher before an expected crowd of 55,000 at Suncorp Stadium on July 2, went five rounds with two Filipino sparring partners and took more than his share of punches.
Philstar.com reported that "Pacquiao was far from the deadly fighter he's long been known for."
Leonardo Doronio, who has just 15 wins in 32 fights, did two rounds with Pacquiao and landed his share of jabs and left hooks.
Then Sonny Katiandagho, who was stopped in Sydney in April by Irish-born light-welterweight Darragh Foley, did three rounds with the world champ.
In some exchanges, Katiandagho got the better of the man who has earned more than $500 million from the sport.
At times it looked like Pacquiao was deliberately letting his sparring partners hit him.
Roach told leading Filipino journalist Dyan Castillejo, of ABS-CBN News, that he has warned Pacquiao not to underestimate the undefeated Horn.
"This guy (Horn) can punch," Roach said. "He's knocked out his last few opponents.
"He's dangerous with the right, he has a real good right hook, and that's something that Manny's been hit with before."
"We're fixing all that now, and it won't be a concern by fight time.
"(Manny) needs to use his speed, his footwork, and his boxing ability."