The Celtic boss is not a good loser - especially when it comes to cup finals...
Interestingly, his 'worst' moment wasn't confronting an opposition player in rage, getting sent to the stands or falling out with one of his star performers.
It was simply losing a match - albeit a cup final.
Indeed, ahead of Sunday afternoon's Scottish League Cup final, the Celtic boss has been discussing how much he dislikes not winning. And defeat to Dundee United this weekend would no doubt be treated with similar contempt by the Norwegian.
RD: "I hate losing. I’ve lost one cup final in my life and it’s maybe the worst thing I’ve done so we will do everything we can to win."— Celtic Football Club (@celticfc) March 12, 2015
Speaking specifically about Celtic's clash with Dundee United, Deila has also been talking up the size and significance of the occasion.
The Bhoys have not won the Scottish League Cup since 2009 and will no doubt relish the opportunity to change that come Sunday.
Deila is therefore keen for Celtic to play at their best and not underwhelm as they have done on occasion this season.
RD: "It’s a huge game. It’s one of our targets and goals and we can achieve it. We’re 90 minutes, hopefully, from a trophy."— Celtic Football Club (@celticfc) March 12, 2015
RD: "If we play bad and lose that’s the worst thing but if we play good we’ve got the quality to win."— Celtic Football Club (@celticfc) March 12, 2015
Celtic are currently six points clear at the top of the Scottish Premiership and will also face Dundee United once more in the Scottish Cup quarter-finals - a replay after having drawn with Jackie McNamara's men last weekend.