Martin O'Neill believes Celtic received a "wake-up call" during last season's title-winning campaign and has warned the Scottish champions they must improve if they are to defend their crown.
Speaking in his first interview since being appointed permanent manager on a one-year deal, the 74-year-old expects another fiercely competitive Premiership race, with Rangers and Hearts both poised to mount serious challenges.
"I think Rangers will get stronger, and Hearts won't go down without a fight," O'Neill told Celtic TV.
"Last season, every team seemed to be getting closer to Celtic than they have in recent years, and that's a concern."
Despite those warnings, Celtic's transfer activity has yet to match the urgency of O'Neill's message.
O'Neill officially took charge on 11 June, nearly three weeks after guiding Celtic to a dramatic Premiership and Scottish Cup double during his second spell as interim manager.
His coaching staff were only confirmed shortly before pre-season began, while the squad remains one of only two in the Scottish Premiership yet to make a signing this summer.
With several departures expected - including uncertainty surrounding Daizen Maeda, Arne Engels and Benjamin Nygren - reinforcements are becoming increasingly important.
Meanwhile, rivals have been far more active.
Hearts have already brought in seven new players, while Rangers, now managed by Derek McInnes, have strengthened with the arrivals of Lawrence Shankland and Scotland goalkeeper Ross McCrorie.
Although Celtic claimed a remarkable 14th league title in the past 15 seasons after winning their final seven matches, O'Neill admitted the achievement has barely had time to sink in.
"If I'd left the club, I probably would have had more time to appreciate it," he said.
"Instead, it feels like we've gone straight back to work."
O'Neill also revealed he initially questioned whether he had the energy to accept the role permanently following the exhausting title run-in.
However, the possibility of regretting the decision convinced him to stay.
One of his biggest priorities is ensuring Celtic qualify for the Champions League league phase after last season's disappointing exit in qualifying to Kazakhstan's Kairat Almaty.
"It's absolutely vital," O'Neill said.
"We need to strengthen the squad and make sure the players are physically and mentally ready for those matches."
He believes missing out on Europe's elite competition last season had a negative impact on the team's domestic form before Celtic eventually rediscovered their best football during the closing weeks of the campaign.
Alongside strengthening the squad, O'Neill intends to reshape the team's style of play.
He admitted he avoided making major tactical changes during his interim spells to maintain stability but believes a full pre-season provides the ideal opportunity to implement his ideas.
The Northern Irishman hopes to combine the possession-based approach of the modern game with the fast, direct and attacking football that defined his successful first spell at Celtic in the early 2000s.
"I want us to keep possession, but always with a purpose," he explained.
With a demanding Champions League play-off approaching in August and another closely contested title race expected, O'Neill knows Celtic cannot afford another slow start - both on and off the pitch.
