Quins kicked off their first season outside the top flight in 18 years of competitive rugby with a laboured 35-9 victory over Bees, one of the league's weaker teams.
Wingers George Harder and Ugo Monye both ran in a brace of tries while full-back Tom Williams also crossed, but it was hardly the high-octane opening expected from the aristocrats of English rugby.
They are heavy favourites to bounce straight back into the Guinness Premiership and while that remains the case, despite an error-strewn display at Sharmans Cross Road, they could find the next eight months trickier than expected.
It was a wake-up call for many of Quins' players and Mehrtens, who produced a mixed display at number 10, admits the club are in for a long winter as their National One rivals target a major scalp.
"Everyone talks about National One being of a much lower standard but I don't think it is. There's probably a big gap between the top Premiership teams and in the second division, but otherwise it there isn't much difference," he said.
"We will be playing teams who want to knock us off every week. Adrenaline can do amazing things to people. We have top make sure our passion and intensity is up to and beyond our opponents."
Worryingly, many of the problems that plagued Quins during their slip towards relegation last season were still evident on Saturday - despite having a summer to work on them under the guidance of a new coaching team.
Their backs saw plenty of ball but this was usually wasted as moves broke down through sloppy handling or poor communication.
All five tries were scored by the back three but overall there was little imagination behind the scrum.
Up front their forwards lacked bite and it came as little surprise when Bees director of rugby Phil Maynard said after the match he was "surprised at the lack of physicality" from the fallen giants.
Maynard's opposite number Dean Richards hoped the disappointing opening was a case of "first-day jitters" and Mehrtens admitted his side had been given a thorough workout by the club formerly known as Birmingham & Solihull.
"It was a pretty torrid game at times - a good hard, battle. We had to work really hard for our tries and we were happy not to concede any. We can take away a lot of positives, not least the acquisition of five competition points," said the Kiwi.
"At this stage of the season every team is disjointed. It does take a while for a team to find its rhythm. It's not always the sides who find their rhythm quickly who do so well.
"I know you need to get points on the board early but you need to peak at the right times as well. You can't peak for 40 weeks. We're happy enough and there is a lot for us to work on."