De Bruyne (34) has left Manchester City for defending champions Napoli, while Modric (39) has joined AC Milan from Real Madrid.
Locatelli insists both midfielders will make a positive impact on Serie A, despite their ages.
He told Sky Italia: "De Bruyne and Modric are two champions, two legends of today's football. It's an honour to challenge them, to face them.
"I've played against them before, and there are so many things I could learn from both of them, but the really beautiful thing is to compete with them."
Lazio legend Paolo di Canio said of the veteran duo last week: "I don't want to be a destroyer, but I have two analyses. The first: I will enjoy their play, but I'm sorry that no one has looked for them in Europe. Not even the third, fourth or fifth forces. We took them from the first.
"We will see the Croatian's creativity or the Belgian's passes, with rhythms, timing and spaces that are easier for them. But Italian football, from Ronaldo to Ribery, brings people to the twilight.
"I want to use a term in a friendly, non-derogatory way: the cemetery of noble elephants. As children, they say they watched our football, but they're still there at 40. They come to wonderful places, but to a tournament that in their memories is the 2006 World Cup, with Cannavaro, a defender, and the Ballon d'Or winner."