Hello Tramontana,
Some 2-syllable adjectives can take both types of comparisons like clever, common, cruel, polite, quiet, stupid, unhappy, (un)tidy, well-known, etc. and likely (when it's an adjective) is one of them although ‘more’ + ‘most’ are more common. I usually use more and most with likely but not always. It sometimes depends on the group I'm with.