De La Hoya's confirmation cleared the doubt about the contest taking place after Pacquaio first announced the contest last month.
When asked why Roach had been ditched - not necessarily permanently - after 34 fights, Pac-Man's adviser, Michael Koncz, told ESPN: "There was some concerns Manny had in the last fight and some statements Freddie made to the media that Manny wasn't very pleased about".
"This is going to be a tough fight".
"It's on! Can't wait to challenge Lucas Matthysse for another world title on July 15 in Kuala Lumpur, Malaysia - July 14 U.S. time #Pacmanisback", his tweet read. Matthysse is also a knockout artist.
The bout see's Lucas defend his world title for the first time after claiming the title in January against Thailand's Teerachai Sithmorseng.The Argentine knocked out Sithmorseng inside eight rounds to claim the then-vacant title.
Beyond a fight with Matthysse, Pacquiao is eyeing a showdown with two-time Olympic gold medial victor Vasyl Lomachenko (10-1, 8 KOs).
Pacquiao also said last week that he would like to square off against 130-pound champion Vasyl Lomachenko if he is victorious over Matthysse.
"I'm the underdog in this fight but I'm used to it".
The 39-year-old Pacquiao lost his WBO welterweight title to Australian Jeff Horn via a controversial unanimous decision in July 2017.
In February, Roach told the Los Angeles Times that Pacquiao has not been returning his calls.