After a run of six consecutive defeats, the hosts now sit in the relegation playoff place, while Portimonense remain just two points ahead and within touching distance of the bottom three.
Nacional suffered a sixth straight defeat in their last game before the international break, as they fell 2-0 away to fellow strugglers Gil Vicente.
The game remained goalless until the 55th minute when Pedro Marques opened the scoring for the hosts, and Samuel Lino would put the game to bed with a goal in the 71st minute to condemn Luis Freire's side to a sixth consecutive loss.
That was enough to trigger Freire's departure after two years at the club, which included promotion to the top flight last season.
He left Os Alvinegros with the club sat in the relegation playoff place with 21 points, while they sit level on points with Boavista and Maritimo in the automatic drop zone.
However, they are by no means condemned to relegation, as they sit just one point behind 15th-placed Farense, while a win on Friday would see them leapfrog Portimonense.
Os Nacionalistas will be looking to begin life without Freire with a victory which would see them move out of the bottom three after their rapid drop down the table.
However, they take on a Portimonense outfit who will be desperate to pick up three crucial points of their own to put more distance between themselves and the relegation places.
Before the international break, Paulo Sergio's side fell to a narrow home defeat at the hands of second-placed Porto.
The visitors took the lead on the stroke of half time through a Lucas Possignolo own goal, but Fali Cande would equalise for Portimonense before goalkeeper Samuel scored Portimonense's second own goal of the match to gift all three points to the defending Primeira Liga champions.
Despite suffering a second consecutive defeat, Sergio should take positives from that game which saw them hold a strong Porto outfit for the majority of the match, other than a couple of defensive lapses.
They will be hoping to bounce back with what would be a crucial win in this game, as they look to move further up the table and away from the Primeira Liga's bottom three.
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Nacional will be without centre-back Julio Cesar, who will serve a suspension after he was shown a red card in the defeat to Gil Vicente.
They could also remain without left-back Witi, who missed the game before the international break through injury after a patchy fitness record this campaign.
Brayan Riascos will be pushing for a return to the starting lineup for this game, after he was recently dropped to the bench despite leading Nacional's scoring charts with five league goals this campaign.
After opting for a defensive shape against a threatening Porto side, Portimonense could revert to a more attacking system on Friday, as they take on a side in the bottom three.
They will be without several players in midfield though, as Pedro Sa will miss the game through suspension after he was shown a late red card against Porto, while Lucas Fernandes has been ruled out for the majority of the season with a ligament rupture.
However, Portimonense will be boosted by the return of midfielder Ewerton, who served a suspension last time out for accumulating five yellow cards.
The line will be led by Beto, who has netted seven Primeira Liga goals this campaign.
Nacional possible starting lineup:
Piscitelli; De Souza, Henrique, Kal, Vigario; Thill, Borges, Koziello, Gorre; Riascos, Rochez
Portimonense possible starting lineup:
Samuel; Moufi, Antonio, Rocha, Anzai; Ewerton, Clemente, Poha; Boa Morte, Beto, Da Silva
We say: Nacional 2-1 Portimonense
In a game between two sides in poor form and worrying situations at the bottom of the table, we just give the edge to the hosts, as they should be boosted by the change of manager.
With six straight defeats, a win in this game is crucial for Nacional, and we see them doing that if they can bounce back from the loss of Freire and kickstart their survival bid under temporary control.