As a guitar teacher I am faced with many guitar-only issues but when it comes to rhythm I can offer a broader perspective. 

Rhythm is just as important to a guitar player or a horn player as it is to a drummer. 

     A drummer must use his sticks or hands to beat a drum in rhythm.  A guitar player uses picks or fingers and hands to strike the guitar strings in rhythm.  I sax player has to press the pads or use their tongues in rhythm. 

     A great exercise to this would be to play with a drummer.  As the drummer plays, mute all of your strings and strum.  You are a drummer.  A flute player is a drummer.  A harmonica player is a drummer. 

     With this said realize that developing a strong sense of rhythm is crucial.  if a drummer can play a measure of 16th notes very smooth, then you should be able to do the same on your guitar.  This is one reasons why some solo acoustic guitar players can make you dance and others leave you lost.  Listen to some solo Dave Matthews or John Mayer and you can hear the rhythm clearly.   

     Buy a metronome and practice playing with it.  Your drumming should be as smooth as the drummer in your band.