The 24-year-old Ecuardorian striker, who scored his first goal at St Andrews in the home win against Burnley on Saturday, is currently on loan from Mexican club Santos Laguna with Blues having the option to buy Benitez when the loan expires.
"I will assess everything in the next couple of weeks," McLeish said after Blues' 2-1 win against the Clarets.
"We have a deadline of 25th May to decide what we want to do. Something could be sorted out before then.
"He has done well for us and we have to consider everything.
"The fee is written into the deal but, you never know, loads of deals have been like that before and have become negotiable."