PSG, which has won its opening three Ligue 1 fixtures, travels to Reims for its final match before the international break.
Messi, signed on a free transfer from Barcelona earlier this month, did not feature against Brest last time out despite having trained with his new team-mates, while Neymar was also absent.
As speculation continues over the future of Kylian Mbappe amid interest from Real Madrid, Pochettino confirmed that Messi, Neymar and the aforementioned France star would all be involved in some capacity on Monday (AEST).
"They will certainly all be in the squad, so we will see if they are in the starting 11 tomorrow," Pochettino said.
"We will assess how he is doing and then see if he is in the squad or not. I haven't given a precise answer yet, so I think it is normal for there to be doubts about that.
"When we sign a player, we try not to have big expectations, but to discover and find out information when we are together. Every club is different so you cannot make comparisons.
"What we are finding out about Leo is that he is a professional and a great player, although we do not need to say that. He is adapting very well to his new club, new team-mates and a new environment."
Asked how Messi will fit into PSG's style of play, Pochettino said: "We are a team that likes to play on the front and keep possession. So, I think that is also what he is used to doing.
"For a player like him, he will certainly play a key role in the French league – regardless of the different attributes that the league has compared to England and Spain – because we are talking about one of the best if not the best player in the world.
"I don't think there will be any issues of him settling in, and he is looking forward to making his debut and helping the team reach its objectives."
With Messi and Neymar waiting to feature, Mbappe opened his account for the season in the 4-2 win over Brest.
The 22-year-old has been involved in four of PSG's past seven goals against Reims in Ligue 1 – two goals and two assists.
While goals had escaped him until last week, Mbappe has attempted (25) and completed (13) the highest tally of dribbles in Ligue 1 so far this term, as PSG looks to extend an unbeaten run on the road in the league to 11 matches.