Having appointed former Wolves boss Nuno Espirito Santo, Spurs will be looking for the Portuguese head coach to guide them to Champions League football for the first time since their fourth-place finish in the 2018-19 season.
Following Jose Mourinho's sacking in April, Ryan Mason steered his boyhood club to seventh at the end of last season to secure a position in the new UEFA Conference League before Nuno's appointment.
Amid continuing speculation surrounding talisman Harry Kane's departure, Nuno may be preparing for life in London without the England captain, who netted four times at Euro 2020 to finish behind Patrik Schick and Cristiano Ronaldo (five each) for the Golden Boot.
But Waddle feels it is the other end of the pitch that Nuno needs to focus on.
"Tottenham need to sort their defence out first and foremost," he told Stats Perform.
"Nuno had a good track record with Wolves defensively. But the football at Wolves wasn't probably what Tottenham fans would want to watch."
Tottenham shipped 45 goals last campaign, two fewer than they conceded in the 2019-20 season.
While Wolves' defence was breached 52 times last campaign – a season largely hampered by Raul Jimenez’s injury and the departures of Diogo Jota and Matt Doherty – Nuno's men conceded just 40 goals in 2019-20.
Up front, the situation with Kane, who finished as the Premier League's top scorer with 23 goals in the 2020-21 season before guiding England to their first major tournament final in 55 years, remains unclear.
With or without their star forward, Waddle feels Spurs have quality in their ranks, which could thrive with a few smart signings.
"[Pierre-Emile] Hojbjerg had a good tournament - good footballer, he's a good player," Waddle added.
"Son [Heung-min] is a good player we know that. It looks like [Nuno] is trying to buy, and it looks like he's trying to get a midfield player [to play] alongside Hojbjerg, which would be good.
"What I see, would it be a top-four team next season? At the present minute before the transfer market really kicks in. I would say well what they've got now - no.
"I would say they're probably going to be sixth or seventh. If they make four or five good signings - possibly top four, yes.
"Maybe they'll go into the pack, and they'll go into it with a chance, but at this minute in time no I wouldn't put Tottenham in top four."