The 32-year-old former Barcelona, Inter and Anzhi Makhachkala striker was reunited with Jose Mourinho, who managed him at San Siro, at the end of August, after the Russian club's owner drastically cut the team's budget.
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"He's looked really sharp in training so far, and so has Willian," Terry told the club's official website. "You speak to them and they talk about how the intensity of their training [at Anzhi Makhachkala] is a lot different to how it is here, so for sure it will take them a few weeks to get their full fitness, but 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."
Eto'o is in contention to make his Chelsea debut when the squad travel to Everton on Saturday.