The Belgian joined the Blues last week in a deal worth up to £58m despite interest from Liverpool.
Lavia became the second big-money midfield signing the club had made in the space of only a few days, following Moises Caicedo through the door at Stamford Bridge.
Midfield had been a key area the club looked to strengthen after a swathe of high-profile exits this summer and there is hope Lavia, Caicedo and Enzo Fernandez will be starting options for years to come.
Caicedo, however, looks primed to start alongside Fernandez for the first time.
“Moises is ready to start, Lavia is not,” said Pochettino on Thursday.
“After [the fitness coaches] assessed him, he needs a few weeks to be ready to be involved with the team. He is reconditioning now and working to try to catch the team and his teammates.
“For Romeo, we need to wait a few weeks until he is involved with the team.”