Lukaku is reportedly set to become the first Everton player to earn over £100,000 ($123,000, 117,000 euros) a week if he puts pen to paper on the new offer.
His agent Mini Raiola who is also the representative of the world’s most expensive player, Paul Pogba — claimed the deal was “99.9percent done”.
However, Koeman said the nature of the modern game sees players rarely honour the whole contract.
“I know they are in talks together about a new contract but if it is 90 percent or 99 percent I need to believe his agent,” said Koeman.
“It is a big deal. The club need to keep their best players and do everything to keep them in the club because that is the future.
“Of course you don’t know in football what can happen in one year but at least there is an understanding that the boy likes to stay and he is a big part of Everton’s future.
“Even when they sign new contracts, they don’t say they will stay until the end of the contract. It is football and business.
“But first of all I think if a new deal is done, that is good news.”
Koeman, who had a good start to his reign at Everton this season but has just one win in his last seven matches, said he wanted to add quality to his squad so that they could challenge higher up the table.
“If you watch Tottenham, they made a step last season, but it is really difficult for them to be close to the top,” said Koeman.
“That seems to be more difficult for Everton. We know we need to battle for players and we have competition of (Manchester) United, (Manchester) City and Chelsea then it is difficult.
“But I still believe we need to get good players to make the next step, and that is what we need to do.”