"They'd be a very difficult rival."
Champions in 2010, Spain will be in the second pot of seeds, alongside England, Colombia, Mexico and Uruguay, when the draw is made in Moscow on Dec. 1.
Four-time World Cup winners Italy, who face Sweden in their playoff match, will also go into pot two should they qualify.
The top eight seeds are made up of the seven teams leading the FIFA rankings - Germany, Brazil, Portugal, Argentina, Belgium, Poland and France - as well as host nation Russia.
Messi, who played for Argentina when they lost the final three years ago to Germany, rated "Spain, Brazil, Germany and France" among his favourites for the title.
(Reporting by Andrew Downie; Editing by Christian Radnedge)
