Alvarez remains Barcelona's top priority this summer, but Atletico Madrid have made clear they will not engage in talks for the Argentine, leaving Barcelona to explore other options in case that pursuit stalls entirely.
Sesko fits the profile Barcelona are targeting if a deal for Alvarez cannot be completed, according to the report, positioning the Slovenian forward as a credible alternative for the club's attacking reinforcements this summer, reports Marca.
The 23-year-old joined Manchester United from RB Leipzig last August in an £85m deal and endured a mixed start to life in the Premier League, scoring just twice in his first 16 appearances under Ruben Amorim before a managerial change midway through the season coincided with a sharp upturn in form.
He finished his debut campaign as United's joint-top scorer in the league with 11 goals, having found a consistent run under Michael Carrick in the second half of the season.
Any move for Sesko would represent a significant outlay for Barcelona given the fee United paid to sign him from Leipzig less than a year ago, and there is no indication United have any intention of selling a player they view as a long-term fixture of their forward line under Michael Carrick.
The situation leaves Barcelona's recruitment department weighing up two markedly different paths forward, with Atletico's firm stance over Alvarez potentially forcing a shift in strategy toward a player who has only just begun to settle into English football.