The pair became friends after signing for Arsenal in 2019 and have remained close.
Brazilian attacker Martinelli considers Tierney one of his closest pals in the sport and cherishes their time at the Emirates together.
Martinelli said: "Yeah, he's my guy. When I got to Arsenal, he came the same year. He's one of the best guys I've met in football.
"He's so kind and so humble as well. So rapid as well! We used to joke who was the quickest in the sessions, in my opinion, it was him.
"He's a great player, he's doing well in Scotland now. They (Celtic) just won the league so happy for him."
Despite their friendship, Martinelli hopes his "guy" has an off-day when Scotland face Brazil on Wednesday.
He added: "I hope he doesn't play so well against us. I'm always cheering for him."
Their relationship will add an intriguing dimension to the World Cup clash at Miami Stadium.
Martinelli will be hoping to get one over on his old pal but is rooting for his success outside of their showdown.