I mentioned that here and here the difficulties of Argentina as they face a tough group. One mistake, Mexico and Poland will be in contention for the knockout stages. Messi has indicated that there's no way they can relax on this group - it has quality. Saudi Arabia is unfortunate to be belong in this group.
On top of that, Argentina needs to top the group as France is expected to top group D too or else if they end up in different top 2 positions in their respective group, they will face each other in the knockout stages.