However, the Argentine superstar has seemingly made a miraculous recovery from a broken arm to be in contention for Tuesday's return date and has travelled with Ernesto Valverde's squad to Milan.
Luis Suarez scored twice on Saturday, the second an 89th-minute victor to complete a late comeback against Rayo.
He added: "The other day [vs Rayo] we started the game very well, but then it was hard for us to play as we did in the previous matches". After six games without a goal, he has now hit six in his last four. During the first game, we did not manage to keep the ball well enough to turn the game around. If I could choose, I hope he doesn't play, ” said Valero. "We know they will score many goals together again".
Messi's absence opened up a gap to be filled further forward and many expected the chance to be given to the 21-year-old Ousmane Dembele.
"Maybe more aggresive because they're at home".
"We have always said that we can play against any opponent".
Only last month, the pressure mounting on Valverde was not incomparable to that felt by Julen Lopetegui at Real Madrid.
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.
A draw against Inter will guarantee qualification from Group B while victory would secure top spot, with two games still to spare.
Despite having that cushion at the top, though, Valverde says his side will still be going all out for the win - a result which would see them qualify for the knockout rounds.
'With Messi, Barca's degree of unpredictability increases but their attitude is the same without him.