Zia is a slogger, he is not opening material. Also Nayeem would be out of sorts being put as an opener, last few months he has been practicing and training to be #4 batsman and he had the patience and temperament to go with it. Putting him in an opening position in an ODI would make him feel unsettled.
"Only ever tell the king what he ought to do, not what he could do; for if the lion knows his own strength, no man could control him."- Cardinal Wolsey