The Godenzonen claimed their 31st Eredivisie crown last term, finishing six points clear of nearest rivals Feyenoord despite having to overcome a string of injury and off-the-field boardroom problems.
However, midfielder Janssen has moved to dispel suggestions that his side are favourites to defend their title, insisting that some new faces are needed to add experience to the young team – not least a replacement for defender Jan Vertonghen, who has joined Tottenham Hotspur.
"Some of the key players from last season are gone, there will be new players who must stand up. This is what I'm worried about," Janssen said to Eredivisie live after Ajax's 4-0 friendly win over Celtic.
“We have to wait and see what happens. We are not title contenders, we need new players. A new, experienced central defender, a leader, for example."
Janssen played no part in the convincing win over the Scottish champions after picking up an injury earlier in the week, but the 30-year-old is hoping to be fit in time for the new season.
"I picked up a problem last Tuesday on my calf, and I’ve only been in training for the last two days," he revealed.
“The coach [Frank de Boer] said he would have preferred me to train properly first before taking risks.
"The goal is to get fit. Then it's a wait until the games get serious.”