Sunday's 3-1 win against Sunderland at Stamford Bridge brought the season to a fitting end for the west Londoners and one of their favourite sons.
Ahead of the match, Drogba announced it would be his final game as a Chelsea player after two spells totalling nine years.
It has been an emotional goodbye and a move borne out of the Ivorian's desire to give youth a chance, with centre-back Cahill confident they have just the man to replace him in Costa.
The 26-year-old's first season at Stamford Bridge has been injury-disrupted, but his 21-goal haul has highlighted what a fine striker they have on their hands.
"I know it's the second time Didier's been here but obviously it's sad that he's played his last game for the club," Cahill said, referring to Drogba's previous exit after scoring the winning penalty in the 2012 Champions League final.
"He said a few words before we left the hotel, it was only brief but it was good.
"I'm sure one day we'll see him back at the club
I don't think he's gone forever but probably from a playing point of view that was his last game.
"They're big shoes to fill but Diego couldn't have started his Chelsea career any better.
"He's had a couple of injuries but nevertheless his goalscoring ratio has been phenomenal.
"He'll be a big player here and with Diego in the team you're a far better side."
Costa will no doubt have an integral role to play if Chelsea are to retain the title next season.
Jose Mourinho's men comfortably won the championship, but the Blues manager has already hammered home the difficulties they face next season.
"He's told us it will be a lot harder to win it next season, he's drilling that into us and as players that's a good thing," Cahill told the club's official website, www.chelseafc.com.
"It's important for us to take his advice as someone who's been there and done it and no doubt it will be tough because teams always want to beat you when you're champions, like Man City have found this season.
"You don't need to be told it's hard
I know how hard it's been this season, even though we've been at the top all the way through.
"I'm sure other teams will strengthen in the summer and that will make it even harder for us."
Source : PA
Source: PA