Holtby has admitted that while it was encouraging to win in Europe, the club's real test will be on Sunday against United, where they will need to be at their best to take anything from the game.
"We did the job in Tromso, won 2-0, kept a clean sheet and got through to the last 32 in first place," he told the club's official website. "That was positive and now we face United.
"It will be a tough game and we'll put everything into it - everything has to be at 100% against United or it will be difficult.
"We've spoken about the City game and we're all determined to show a different side of the team, a different face. We're capable of winning this game and we'll give everything."
Tottenham are currently ninth in the Premier League table ahead of this weekend's fixture.