The former Blues captain was blown away by the power and strength of one player during a visit to the club's training ground.
Despite his wiry frame, Cole Palmer emerged as the standout performer during a bleep test drill.
Terry told talkSPORT: "He's quite quick. I was at Chelsea the other day, and they were doing a bleep test.
"He won it by a mile. He was cruising, like people were dropping out. He was there right to the end.
"In terms of fitness levels and the power, he looks quite skinny, but very strong.
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"Takes the ball on the half-turn really quick as well. Like, impossible to play against."
Palmer's slight figure often allows him to evade defenders when dribbling with the ball.
Terry even believes the 22-year-old midfielder makes it look 'too easy' when he is on the pitch at Stamford Bridge.
The retired defender added: "It seems to be too easy for him at times. He strolls through games a little bit, but he's so far ahead of everyone else in terms of the way he sees the game. He always plays that forward pass.
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"He's never looking for the simple one. I think him and [Nicolas] Jackson have a really good relationship as well.
"Nico likes to play on people's shoulders, which a lot of strikers don't do nowadays as well. But he's just been phenomenal.
"When you go to these games at the Bridge you can't help but just watch and follow him. The ball's at the other end of the pitch, and you're just watching his movement.
"He's always on the half-turn, getting in little pockets.
"(Gianfranco) Zola used to find himself in areas that he didn't even know existed. He's the same as well, but he turns and wants to drive at people and play forward, which I think is the key to it."
Palmer has established himself as the Blues' key man since signing from Manchester City in 2023.
After winning the PFA Young Player of the Year award in his debut season, he penned a new nine-year deal back in August.
The England international has since gone on to score 14 goals and provide six assists in the Premier League this season.
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However, Palmer was forced to miss last weekend's defeat to London rivals Arsenal through injury.
He continues to be assessed by the club on the muscle issue picked up in training.
Enzo Maresca is hopeful Palmer will be fit to return to action after the international break, with the midfielder a doubt for the Three Lions.
Palmer had been included in Thomas Tuchel's squad to face Albania and Latvia in World Cup qualifying.