Simunovic made an impressive debut for the Bhoys against Ajax in the Europa League, but he has not been seen since.
His absence from Celtic's 2-0 win over Raith in the League Cup on Wednesday was particularly surprising, but Deila insists that is not concerned about the 21-year-old's fitness
"He's twisted his ankle," Deila said. "He's getting better every day, it's not big, but he was not 100% tonight."
It remains to be seen whether Simunovic will be back for Celtic's game this weekend, which comes against Hearts on Saturday.
Deila will be hoping to see him return to fitness sooner rather than later, though, as he has made a good first impression at the club.
The Croatian youth international looked assured in possession against Ajax, while he also showed his defensive solidarity.
He does face a fight to win his place in the team back, though, as Celtic have scored eight goals in their last two games, without conceding, and look to be hitting their groove.