Reja was appointed as Vladimir Petkovic's successor earlier this month and presided over a 1-0 Serie A win against Inter in his first game back as coach at the Stadio Olimpico.
The victory leaves Lazio 10th in Serie A and Reja is not against the idea of improving his squad with new additions, provided opportunities arise to do so.
"The squad is competitive, what counts is to then see in the official games how the players behave," he said.
"They will have to show me their value on the pitch. I’ve seen some great qualities, to play for Lazio you need to have personality.
"We are evaluating various opportunities, if there is the possibility we will make some moves. We will wait this week, after the game against Parma, then we will evaluate the situation.
"The will is to bring in an important player, of substance."
The likes of Napoli's Paolo Cannavaro and Juventus striker Fabio Quagliarella are two names reported to be of interest to Lazio.
Reja explained how highly he thought of the pair, but stopped short of confirming his interest in signing them.
Asked whether he would welcome either at the club, he said: "Maybe, they are two excellent players.
"I trained Cannavaro at Napoli and I have always held him in high esteem, he was the real captain of the squad.
"They are both players I really rate. Quagliarella is a player of value, a second striker who also moves around the primary attacker.
"I also read about (Milan striker Alessandro) Matri, these names all come out."
