The Norway international was surprisingly pitched into the first-leg of the Champions League third qualifier against Legia Warsaw in Poland last month by manager Ronny Deila only a couple of days after signing a six-month loan deal from Cardiff.
The 23-year-old forward looked out of sorts along with his equally hapless team-mates and was eventually substituted after an hour of the 4-1 defeat.
He was left out the starting line-up in the next two games but returned against United where his brace washed away his Warsaw woes which he put down, in part, to not knowing the names of his team-mates.
"As a footballer you always want to play," said Berget. "The manager asked me and I told him I was ready for it, and I was.
"I don't think anybody had their best game there (Warsaw).
"So it is easy to say the new guy had only trained a couple of times, but things could have gone the other way as well.
"I thought I was ready for it but of course it is easier when you know the names of the players you play with."
Berget certainly looked more comfortable against United, defensively hopeless as they were, and indeed he was only thwarted by the crossbar from grabbing a hat-trick.
The former Stromsgodset player said: "It's been long time since I have scored twice.
"It was the second game I have started, I feel I'm getting into it. I know all the boys now and I've settled well.
"I knew Stefan (Johansen) from before so that is an easier way into the group but the boys have been good, all of them, so I've settled in fine.
"I have already played more games here than I did in Cardiff so that's a good thing, and scored two goals so that is great.
"You can't say I've proved myself with one game but it is definitely a step in the right direction and hopefully I'll get more goals."
It is back to the drawing board for Jackie McNamara's side who contributed massively to their own downfall.
Their troubles started in the third minute when Celtic debutant Jason Denayer, the 19-year-old Belgian defender who arrived on a year-long loan from Manchester City, put the home side in front when United failed to deal with a Kris Commons corner.
Further goals by Kris Commons and Stefan Johansen, both also avoidable from the visitors' point of view, had the points tied up by the interval and stand-in skipper Charlie Mulgrew claimed a fourth with a nick although that was disputed by Anthony Stokes who curled in the free-kick.
Berget scored either side of a deflected drive by United captain John Rankin which left McNamara wondering if the early goal had frightened his team.
The former Hoops defender said: "Maybe losing a goal after three minutes spooked a few of our players who have been on the end of it before. A bit of fear creeps in."
He added: "But I'll make sure all of the players watch footage of the set-pieces and the goals we conceded."