Tanner admits when they snapped up the Figueirense attacker almost ten years ago, his fitness stats were nothing short of shocking.
He declared: "I went over to Brazil twice that year, but it wasn't easy to get the right impression on Firmino during a game because he didn't actually play regularly.
"With the World Cup being held that season, they had a break in the summer, and you know how tight the national calendar is over there, which roughly means that they play Saturday, then Wednesday and then Saturday again.
"In between, they need to travel, so it was very hard even to follow him in training, but we managed to get into the stadium facilities as tourists to watch him working.
"There was one specific session where I said, 'OK, this was the right type of player.' He's bringing along the right mentality, he's eager to learn, he works hard - what we saw in training was far better than in game. That was the reason why we took that decision. We had also sent a scout over, but his report was negative on him."
Despite Firmino convincing Hoffenheim of his talent, the German club did have major concerns about his fitness during his early days with the club.
In fact, Tanner says that the Brazilian posted some of the "worst numbers" seen in endurance tests, labelling the results "worse than my grandma".
He told the Bleacher Report: "You should have seen the data that we got on him when he was coming for the first time [to Hoffenheim]. In Germany, we usually do these endurance tests, blood ones, which are pretty accurate, you know, and he had the worst numbers I have ever seen in professional football.
"I would say to emphasise that he was even worse than my grandma. You can't imagine. They were so low that you couldn't even believe that he was ever able to play professional football."