Lukaku joined Everton from Chelsea 10 days ago in a club-record deal worth £28 million after a successful loan spell on Merseyside last season.
The Belgium international scored 16 goals last term, helping Everton to a fifth-place finish in Martinez's maiden season in charge.
He then scored in Belgium's 2-1 last-16 win over the United States at the World Cup as Marc Wilmots' men reached the quarter-finals in Brazil.
And, ahead of Everton's opening-day trip to Premier League new boys Leicester City next Saturday, Martinez is wary of risking the 21-year-old former Anderlecht man.
"When you have players coming back from the World Cup you have to decide if you want the players straight away or do you give them a bit of time to refresh and be ready to play for 10 months?" Martinez said.
"It will only be [Belgium winger] Kevin Mirallas and Rom who might not start the season. For Romelu, it will depend on how he adapts to work from now until August 16.
"There is a degree of risk if you put them into a situation so early. They've been training really well and are accumulating a lot of work."
Martinez is willing to dip into the transfer market again, but stressed that Everton's remaining business will depend on the fitness of Arouna Kone and Bryan Oviedo - both of whom are recovering from long-term injuries.
"Towards the end of last season I said we needed six more players, but it wasn't six new players, it was six players," Martinez added.
"We have added Barry, Lukaku and Besic and we also have [full-back] Tony Hibbert who re-signed.
"We still need one or two, depending on how quickly Arouna Kone and Bryan Oviedo recover from their injuries."
