Disappointed: Thomas Frank
Tottenham Hotspurs Football Newsletter
Your matchday briefing on Tottenham, featuring team news and expert analysis from Matt Verri
I would like to be emailed about offers, event and updates from Evening Standard. Read our privacy notice.
Spurs played out their first goalless draw in 125 matches but could easily have been beaten in the Champions League.
Monaco had several big chances, particularly in the second half, and Guglielmo Vicario had to produce eight saves to earn his side a point.
It was a poor response from Spurs to their Premier League defeat to Aston Villa on Sunday and Frank was left frustrated by his side's display.
"The intensity was not at the level we want to for whatever reason," Frank admitted on TNT Sports.
"I think it it's something we've been very good to play with basically every game more or less. Today we couldn't go for whatever reason. That can be momentum, it can be a lot of things.
"Then I think the general level we made too many mistakes. Decision-making and simple passes that didn't help on the day."
Spurs were also fortunate to pick up a point in their last Champions League match against Bodo/Glimt, when they came from two goals down to draw.
Read More
Three things we learned from Chelsea win as Blues hit ruthless streak
Eintracht Frankfurt vs Liverpool LIVE: Latest score and updates from Champions League
Liverpool knock five past sorry Frankfurt in Champions League rout
Boost your child’s education to help them excel
They have five points from their first three European games but have not yet produced a convincing performance in the competition.
Frank insisted it was a point gained rather than two dropped, even if he did accept that Spurs were second-best at both ends of the pitch.
"It was a hard point won," Frank said.
"I think it was a relatively even first half. Second half our performance was not good and Monaco were better than us.
"It demanded a lot to keep the clean sheet which we did in the end. I think Vicario was very good but we didn't hit the performance level we wanted to, especially in the second half."
He added: "We're definitely learning [in the] Champions League. Playing away in the Champions League is always difficult.
"We still need to defend a lot but on the flip side, I think we created some chances, but of course I want more."