Juve – who lost last season’s Champions League final to Real Madrid – visit Barca in Tuesday’s Group D opener.
Brazil forward Neymar left for Paris St-Germain this summer after his 222m euros (£200m) release clause was met.
“Along with Real Madrid they are favourites to win the Champions League,” said Allegri.
“They’ve lost Neymar and replaced him with a youngster in Ousmane Dembele – but they remain, along with Real, one of the two strongest teams in the world.”
Juventus beat Barcelona 3-0 on aggregate in last season’s quarter-finals. The manager thinks a third clean sheet against them this season would be “extraordinary”.
“We know that playing three games in a row against Barcelona – considering the two of last year – without conceding a goal, would be very difficult, even if Juve could do it.”
Barcelona have won their opening three La Liga games under new boss Ernesto Valverde.
“We are ready to face an exciting competition and to start with a win means a lot in such a short league,” he said.
“Juventus are a powerful team that distribute the ball well from the back and who try to press high. They have a great coach and great players, all of whom must be taken into account, not just [Argentina forward] Paulo Dybala.”
