Barca, a club grappling with financial problems while also attempting to keep six-time Ballon d’Or award winner Messi, will elect a new president on Sunday from three candidates – Joan Laporta, Victor Font and Toni Freixa.
Victory was Barcelona’s second since the club’s Nou Camp offices were raided on Monday and former president Josep Maria Bartomeu was arrested in a probe related to allegations of improper management and business corruption.
Osasuna, a side 11 places adrift of Barca, were first to threaten, with Ter Stegen at full stretch to push Barja’s effort wide.
A brilliantly threaded pass from Messi and fierce finish from Alba into the roof of the net put Barca ahead, but it was Ter Stegen who ensured they maintained that lead after foiling Garcia.
Osasuna worked hard to limit Barca after the break, but they failed to find an equaliser before substitute Moriba opened his Barca account with a fine curled finish.
La Liga leaders Atletico Madrid, who were 10 points clear of Barca at the end of January, host third-placed city rivals Real Madrid on Sunday.