"We're not able to fight Amir Khan because of Ramadan, so we decided to take this fight in the interim," Pacquiao's adviser Michael Koncz said in comments published by News Corp media.
"We signed it, so yeah, we're excited about it. But again, my preference was to fight Amir Khan. That didn't work out. We had this on the table so we decided to take it."
Eight-division world champion Pacquiao agreed to an April fight against Briton Khan in the United Arab Emirates but his promoter Bob Arum said last month it had been postponed after the $38 million offer for the bout failed to materialise.
The July 2 fight against Horn will be the 38-year-old Pacquiao's first since his unanimous decision victory for the title over American Jessie Vargas in Las Vegas last November.
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"Manny will train and do the best he can and hopefully we get the fight over with very quickly, we get out of there, we say 'hi' to the Australian fans and media and come home," Koncz said.
"Frankly and honestly, and nothing against Jeff Horn -- I've never met the kid, I don't know him personally -- but the name recognition is I guess why the fans picked Amir Khan."
In February, Philippines senator Pacquiao (59-6-2) asked fans on Twitter who he should fight next in a four-man poll and 48 percent of the respondents voted for Khan.
Horn (16-0-1) won seven percent behind Briton Kell Brook (24 percent) and American Terence Crawford (21).
(Reporting by Ian Ransom; Editing by John O'Brien)
