If, in the depth chart, I put a MLB in the WLB and SLB positions in supplement to his one (to train him as a versatile backup), will he learn slower than a MLB who "just" learns his position? Does it depend on intelligence ( an intelligent player being more versatile)? Thanks!

Yes and yes - the assigned position is what the player practices at during the week so they advance in their experience even without playing time, but they do gain experience for each play they are on the field at a position during a game, so they do learn if they are playing out of position. The higher their intelligence, the faster they learn the position.

When you say "assigned position" you thus mean the positions the player is allocated to on the depth chart?

Change Position

A player's assigned position determines what areas he works on during practice, regardless of where he might be in the depth chart. Also, when you let your coaches set your depth chart, he will only place players into the depth chart where their position matches. Below you will see the current ratings for each position for this player. Choose a position from the list below to assign this player to the new position