The Norwegian says some of his younger players are not mentally strong enough yet.
Deila's troops sealed a fifth successive Scottish Premiership title on Sunday by beating Aberdeen 3-2 in a game which saw 19-year-old Manchester City loanee Patrick Roberts score twice at Parkhead.
Eighteen-year-old left-back Kieran Tierney also started, while Saidy Janko, 20, earned a 20-minute cameo.
The Bhoys went 3-0 ahead, with Mikael Lustig adding a third shortly after half time, but Derek McInnes's side got back into it through goals from Niall McGinn and Andrew Considine.
Indeed, the Dons built up plenty of momentum at one point, and Deila says that his slightly more inexperienced figures need to go into their shells a little less when Celtic aren't on top.
"They've been given a lot of experience in my two years here. You can see that they need to be mentally tougher, stronger," he is quoted as saying by Soccerway.
"When things are going a little bit against you, like today, they sometimes go into their shells. That is not positive, we need to get out of that and get the best qualities like we did in the first half."
Deila will leave Parkhead this summer after failing to earn a new deal, but he leaves a very decent legacy in the form of two league titles and the likes of Tierney and Roberts, whose loan deal expires at the end of next term.