The La Liga leaders confirmed on their official website on Monday that the 26-year-old would be sidelined for seven days after spraining ligaments in his right knee during Sunday's 4-1 victory over Real Betis - in which he scored a brace.
The Catalan midfielder was substituted 10 minutes from time at Benito Villamarin and he told fcbarcelona.com after the game: "I wasn't comfortable, and seeing that the match was beyond Betis' reach I preferred to be subbed out."
Fabregas underwent tests on Monday to determine the extent of the injury and is now set to miss the world and European champions' friendlies against Equatorial Guinea in Malabo on November 16 and three days later against South Africa in Johannesburg.
This will be Bartra's first involvement in the senior set-up after featuring 16 times for the Spain U21 side - including winning the European Championship this summer and 2011 - and the 22-year-old centre-back could make his debut on Saturday or next Tuesday.
Bartra received a message of support ahead of his step up to the national team, as Barca captain Carles Puyol wrote on his Twitter account: "Congratulations Marc you deserve it. I'm happy for you. Take the number 5."
The defender joins club-mates Victor Valdes, Sergio Busquets, Andres Iniesta and Pedro in the Spanish squad.