Mascherano is due to take charge of his first Inter game in the Concacaf Champions League round of 32 tie against Sporting Kansas City.
However, due to the freezing conditions, it was rumoured that Messi would not feature because of the conditions.
But Mascherano insisted: "I can assure you that Leo will play. He is 100% available.
"The line-up and the plan will not vary much. The reality is that we will try to be protagonists and try to go in the same line that we have had in the pre-season, although the weather factor can condition our game, but we will try to do it in the best way and we are going with the intention of getting a result that will lead us to the knockout stage."
Inter will meet Sporting on Tuesday night.
Mascherano added, "The team has been doing very well on a defensive level, or at least we are satisfied, obviously with the idea of continuing to improve."