Messi missed the Catalans' season opener last weekend through a calf complaint, with Athletic Bilbao claiming a late 1-0 win in his absence.
Much to Rubi's dismay, the Argentine attacker has returned to full training and will likely play some part against Betis.
While he accepts Messi could well start the contest, Rubi has his fingers crossed a flu might just strike him down and give his side one less thing to worry about.
“I think Messi is going to play from the start. It’s what I think,” Rubi said.
“I’d prefer Messi to have a cold, not an injury, but a cold so he cannot play.
"We have to go with a point of courage but knowing that any player they put out will be good.
"If you go with excessive respect you are losing 1-0. You have to be brave but not commit nonsense, because they will take advantage of that."
Betis began their La Liga campaign with a 2-1 loss to Valladolid having finished last season in 10th where they did claim a surprise 4-3 win over Barcelona in November.
Rubi, who joined the club in June from Espanyol, was asked to weigh in Barca's protracted pursuit of Neymar and declared it can only help the Catalans to have such a player.
"I am nobody to say if Barcelona should sign Neymar or not, but I have worked with him and he is one of the five best in the world," he said.
"Having one of the five best in the world is wonderful."
While the Catalans remain in a transfer battle for the Brazilian, Ernesto Valverde will be hoping his side can bounce back quickly from their shock loss to Bilbao.
Barcelona eased to the La Liga title last season, finishing 11 points clear of Atletico Madrid, and are now looking to claim the Spanish top-flight for a third straight season.
With Atletico and Real Madrid both claiming wins in their opening games, this is the first time Messi has ever seen his side trail both by three points.