Sergio Conceicao's side have lost two of their opening six league games to slip off the pace at the top of the table, while their visitors have picked up only four points so far and head into the weekend sitting second from bottom.
Just six games into the new season and already Porto find themselves six points off Sporting Lisbon and five behind Benfica - by no means an unassailable lead but certainly a difficult one to recover from in the Primeira Liga.
Two defeats in their last four games - at home to Maritimo and away to Pacos de Ferreira last time out, both by a 3-2 scoreline - means that for the remainder of the campaign they are likely to have little margin for error.
Porto dropped only 20 points throughout the whole of last season on their way to the title, but have surrendered eight of the last 12 available with a win, a draw and those two defeats in their last four outings. Indeed, they have now lost four of their last 16 top-flight outings, which is as many as they had lost in their previous 52 before that.
It makes for their worst start to a season in a generation, and this weekend they are in danger of losing back-to-back league games for the first time since April 2016.
Losing to Portimonense would be the biggest shock yet, though, and Conceicao's side did return to winning ways with a comfortable 3-0 triumph over Marseille in the Champions League on Tuesday night.
Portimonense can only dream of the heady heights which constitutes a poor start to the season for Porto, and they once again look destined for a relegation battle this season.
Paulo Sergio's side were only spared demotion last term due to an administrative error on the part of Vitoria de Setubal, who actually finished one point and one place above 17th-placed Portimonense but were relegated to the second tier as punishment.
Portimonense once again find themselves 17th heading into this match having lost four of their last five games and only picked up four points from the 18 on offer so far.
They will know that improving that record away to Porto is unlikely, but they only narrowly lost the corresponding fixture last season and so may harbour hopes of getting something from a Porto side that has struggled to convince so far this term.
Porto Primeira Liga form: WWLDWL
Porto form (all competitions): DLWWLW
Portimonense Primeira Liga form: DLLWLL
Out: Emmanuel Hackman (hernia)
We say: Porto 2-0 Portimonense
On paper this looks like a comfortable win for Porto, but they have not been in great form themselves and Portimonense do not tend to lose by big scorelines - they haven't lost by more than two goals since January. It would be a surprise if the visitors did get anything from the game, but we also cannot see Porto running riot.