The 36-year-old has looked at his talismanic best in recent outings, after following up goals against Maribor and Manchester United by netting an equaliser in Tuesday's League Cup win over Shrewsbury Town.
Consequently, Cahill says that Drogba - who scored 157 times in his first spell at Stamford Bridge - has merely picked up where he left off since leaving the Blues in 2012.
"In the last two games he is fully fit now, he's looking sharp and dangerous - exactly like he was when he went. He was massive for us on Tuesday and he got a vital goal," Cahill told BT Sport.
"Who knows how long he can go on for? That's for the club and Didier to decide but at this moment we've started the season well and he's been a big part in that.
"What a character he is off the pitch. Obviously Diego [Costa] is playing the majority of the games but Didier is getting back to match sharpness."