Barca lead Group B with nine points from three games heading into Tuesday's fixture in Milan, ahead of Inter (six points), Tottenham (one) and PSV Eindhoven (one).
In Serie A play, Inter is coming off a 5-0 win over Genoa.
Barca is in first place in the group and beat Inter 2-0 last time out without Messi.
Luis Suarez scored twice on Saturday, the second an 89th-minute victor to complete a late comeback against Rayo. We expect them to have a little more pressing and pressure high up the pitch.
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Barca will be without Thomas Vermaelen for the trip to Milan, while Lionel Messi and Samuel Umtiti are doubts.
However, hours later the club announced the travelling squad for the match on Twitter, with his name included - claiming the medical team has given an "all clear" verdict.
Mauro ICARDI scored for Inter in their 1-1 draw against FC Barcelona in the Champions league. Umtiti has returned to training but is still not fully match fit yet, meaning Gerard Pique and Clement Lenglet will start at the heart of the defence once again.
Rayo Vallecano fans were in good mood during the last seven minutes of the game thinking their team would record an emphatic win over Barcelona.
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Ivan Rakitic, Sergio Busquets and Arthur are expected to make up the midfield.
Upfront, Luis Suarez is another player who has stepped up in Messi's absence, and should be focal point of the Barcelona attack.
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Moreover, Pep had praise for a certain Saints player's dead-ball technique. The goal takes AGUERO joint top scorer on the league with seven goals.