Wales international Bale has agreed a 12-month deal to move to Los Angeles after his contract expired with Real Madrid.
"Well this league is very physical," Vela told reporters.
"If Bale is in good shape and comes with a good mentality and attitude he will be very important because he is a big player that comes from Real Madrid so I don't have to talk much about him.
"Everyone knows how good Bale is.
"The only thing I can say is that he will have to work hard because MLS is not an easy league like many people think."