Both players signed from Anzhi Makhachkala late in the transfer window and could be given their first taste of English football at Goodison Park this Saturday.
Terry, who has started all three league games for the Blues this season, is thankful that he is now playing with Eto'o rather than against him.
"You speak to them and they talk about how the intensity of their training is a lot different to how it is here, so for sure it will take them a few weeks to get their full fitness," Terry told Chelsea TV.
"Once they do they'll both look very good. I said to [Eto'o], 'Thank God you're here', because I've had so many battles against him.
"He's such a threat, he's obviously a few years older but he still looks very sharp and hungry. He's got an eye for goal and I think he'll be a good addition. He was welcomed straight into the squad."
Willian scored twice against Chelsea in a Champions League group-stage encounter at Stamford Bridge last season.