The England defender also believes Diego Costa will find it hard to force his way back into Jose Mourinho's side at Stamford Bridge.
Drogba, 36, scored in Chelsea's narrow 2-1 win at Shrewsbury on Tuesday to fire them into the quarter-finals of the Capital One Cup and team-mate Cahill believes the Ivory Coast star is on top form after his summer return following two years away
He said: "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
"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 is playing the majority of the games but Didier is getting back to match sharpness
Loic (Remy) scored but came off so to get them all back fit and scoring goals is vital for us."
Drogba has scored 160 goals in two spells for Chelsea and won the league three times, four FA Cups and the Champions League in 2012 before he left that year.
Costa is expected to be fit for Saturday's Premier League visit of QPR after suffering with a virus and hamstring and groin problems, b ut Cahill insisted the nine-goal hitman is not guaranteed his place back following Drogba's recent form
The centre-back said: "There are options, not just up front but all over the pitch.
"We have turned up on Tuesday with some fresh legs and some of the lads who played (at Manchester United on Sunday) and we knew it was going to be a tough game but we grafted and showed character and got the right result in the end."
Chelsea will move nine points clear of Manchester City with victory against Rangers with third placed City hosting Manchester United at the Etihad on Sunday.
But Cahill said the leaders must not take three points for granted against second-bottom QPR.
"We have a chance to get the right result and hopefully sit back and enjoy Sunday but it's never easy, you have to prepare right, rest up and make sure we're ready," he said.
"The onus is on us to be on the front foot, we need to prepare well and make sure we're fully focused.
"Last season we struggled at times against teams people thought we were going to beat
It's never as easy as that."
Source : PA