Both the second-half substitute and Adam Matthews, who was scratched from the line-up just before kick-off with a calf problem, are expected to miss the first leg of Celtic's Champions League play-off against Maribor in Slovenia on Wednesday.
Forrest pulled up suddenly as he made a darting run down the pitch and the manager admitted the early prognosis did not look good. "I don't know [the extent of the injury] but that was a negative end to a positive day," Deila said. "I hope it is not serious but it didn't seem good when he walked off the pitch. I will see but I think [he will miss the Maribor game]."
Injuries aside, Deila was happy with the way his team played on the occasion of his first match in charge at Celtic Park. "I am into performances and today was a very good performance," he said. "For 60 minutes it was very good with pressing and transition. Our passing was much better and we created a lot of things."
Dundee United manager Jackie McNamara, in contrast, admitted the result was "embarrassing" for his side and lamented the poverty of their defending. Four of Celtic's goals came from set-plays.
"I was confident coming to the match today as I felt we could not just participate, but win it," McNamara said. "But if you don't do your jobs properly, the fundamentals, then you're going to get punished. To lose four goals at set-pieces is especially disappointing."
Former Celtic owner Fergus McCann had unfurled the league flag prior to kick-off and described the moment as "an honour". "Things go up and things go down but Celtic we hope will go on forever," he told an appreciative crowd.