The Turin giants notably sold midfielders Andrea Pirlo (New York City FC) and Arturo Vidal (Bayern Munich) and striker Carlos Tevez (Boca Juniors) over the summer.
But the 'Old Lady' has responded in style, adding to the capture of Sami Khedira (Real Madrid), on a free transfer, Paulo Dybala (Palermo) and Mario Mandzukic (Atletico Madrid) by signing highly-rated Brazilian defender Alex Sandro from FC Porto on Thursday.
Marotta said another "top midfielder" could join the club prior to the summer transfer deadline but in the meantime he is "optimistic" Juve will again be the team to catch this season.
"The departures of Tevez, Pirlo and Vidal were obviously significant, but there was nothing we could do," Marotta told La Gazzetta dello Sport.
"But we've taken stock of the situation, and acted accordingly."
Juventus (twice), Torino and Inter Milan are the only teams in Italy to have won five consecutive league titles.
Marotta added: "I don't know if we're stronger or weaker than we were last season, but Juve is Juve and we can't be anywhere else but on top.
"Personally, I'm very optimistic."
Juventus's fourth straight league trophy last season was complemented by their first Cup triumph in 20 years and their first appearance in the final of the Champions League since 2003.
Although their dream of an unprecedented club treble was kept in check by Barcelona in Berlin, their achievement in reaching the final gave a massive shot in the arm to an ailing Serie A as well as boosting their own reputation as a team to be reckoned with among Europe's elite.
Roma, runners-up the past two seasons, have every right to envy Juve's honours list having won their last scudetto in 2001.
Yet Malian midfielder Seydou Keita believes Roma, for the third year in succession, can emerge as Juve's biggest challengers.
"Juventus remain favourites, and it can't be any other way given they've won the last four titles," Keita told Sky Sport.
"But if we manage to keep the consistency that we lacked last season after the winter break, we can push them from then until the end of the season."
Roma coach Rudi Garcia went a step further, stating that his side will go on to beat the Serie A record of 10 consecutive victories from the start of the season that they set in 2013-2014.
It was a bold prediction, given Roma welcome Juventus - who finished the season with a 17-point cushion -- to the Stadio Olimpico on August 30.
"The season will be long, but it will be tighter than last year," Garcia said.
Having sent ineffective Ivorian striker Seydou Doumbia back to CSKA Moscow, Roma have been boosted by the arrival of Bosnian striker Edin Dzeko and lively Egyptian winger Mohamed Salah, both on loan deals from Manchester City and Chelsea respectively.
Lazio, who host promoted Bologna on Saturday, finished strongly last season to finish third and are battling Bayer Leverkusen for entry to the Champions League group stages.
AC Milan and Inter Milan have been woeful the past two seasons and, without the burden of playing in Europe this season, are desperate to make amends with credible campaigns that will secure Champions League football next year.
Bologna, returning to the top flight after a season's absence, have been boosted by the signing of former Roma and AC Milan striker Mattia Destro.
Fellow promotes Frosinone and Carpi are at home to Torino and away to Sampdoria respectively.