Watt had run-ins with former Celtic managers Neil Lennon and Gordon Strachan, and spent last season on loan at Belgian outfit Lierse where he was banished to the reserves after a falling-out with boss Stanley Menzo.
The 20-year-old has reportedly attracted interest from Standard Liege, but Deila insists that he is prepared to give Watt a chance to prove himself.
He is quoted as saying by the Daily Record: "He needs to play but it's all about attitude. It's about being prepared every day, what you do on the pitch, in training, showing your best. I don't give away minutes, you have to earn them. Every player in pre-season will get minutes and then we'll see.
"Tony has to do a lot of work on those things, he needs to develop."
Celtic will begin their Scottish Premiership defence with a home game against Partick Thistle on Saturday, August 9.