Firpo, who joined Leeds from Barca, played with Raphinha at Elland Road before he left two summers ago.
"Raphinha's current form doesn't surprise me. The season I played with him at Leeds, I could already see the signs of a great, great player," Firpo told MARCA.
"Physically, above all, he was incredible. He had high-intensity distance data, sprints per game... that I'd never seen before.
"This season's numbers are stratospheric . He's doing things that make me very happy for him. Maybe he didn't know he could reach this level.
"With those physical abilities, being technically good and at Barcelona, surrounded by the best, it was crystal clear that he was going to grow as a footballer.
"This season he's found a healthy dressing room, he's one of the captains and Flick has trusted him from the very first minute."