The clash with Deportivo Alaves on Sunday (AEDT) may prove to be Sergi's only game as boss of the first team, with Barcelona eager to bring Xavi to the club.
Sergi's regular role is as coach to Barcelona B, and he has had precious little time with the senior stars since Koeman was dismissed on Thursday (AEDT), after a 1-0 defeat to Rayo Vallecano.
Barcelona heads into the weekend in a lowly ninth position, nine points behind leader Real Sociedad, albeit with one game in hand.
It has played the same number of games as Real Madrid, which sits six points clear of Barcelona and already has a Clasico win in the bag this season.
Sergi believes Barcelona can get back on track and said in a news conference on Saturday (AEDT): "We have to be united, people have to come to the ground tomorrow.
"The way of playing will not change, but maybe some tactical concepts could provide the solution. The style is non-negotiable."
He added: "Tomorrow we have to win the game because LaLiga is open. We have to recover the idea of vertical football."
Former Barcelona and Spain defender Sergi will be without Ansu Fati for the game because of the teenager's knee injury, but Ousmane Dembele could be involved for the first time this season, with the winger back in training after his own knee lay-off.
Sergi said: "I want to put some joy back into the team and get them enjoying their football again. I need all the players to be plugged into turning this situation around."
Playing a vertical game, which would mean being more direct, would seem an obvious move given the personnel available to Barcelona.
Feeding the ball as early as possible to the experienced and capable likes of Memphis Depay and Sergio Aguero may be the smartest option for a team who have struggled to find an identity since Lionel Messi's move to Paris Saint-Germain.
Barcelona have played fewer long passes (449) than any team in LaLiga this season, according to Opta data, but they have played the most possessional passes (6,364), as well as the most passes in the opposition's half (3,431).
Clearly, they are struggling to find a winning formula with their keep-ball game. This has not been helped by their finishing, with just 15 goals scored from an expected goals (xG) total of 18.7.
While Barcelona have lost their last two LaLiga games, to Real Madrid and Rayo, Alaves have picked themselves up after taking just three points from their opening eight matches and have beaten Cadiz and Elche back to back.
Barcelona have not lost any of their last 20 games at home to Basque opponents in LaLiga (W18 D2) since their 2-1 loss against Alaves in September 2016.
Alaves have kept only one clean sheet across their 16 away games against Barcelona in LaLiga, conceding 53 goals across those games. The shutout came in a shock 1-0 win in February 2000.
Sergi said of his opportunity to lead the Barcelona first team: "First of all, I would like to thank the club for the trust they have placed in me. Secondly, I want to send a big hug to Koeman because we have worked side by side these months.
"I arrive in a complex situation, but we are already working. We will have to look for the opponent's weaknesses and try to break their streak with our game."