Zungu scored a late goal to seal Bafana Bafana's 2-0 win at Moses Mabhida Stadium on November 13 but he returned to Scotland with a knock.
The Amiens loanee has only played a match for Rangers since his arrival in October, and he is yet to play in the Europa League.
Ahead of Thursday's match at Ibrox Stadium, Gerrard disclosed how long Zungu will spend on the sidelines, however, a decision is yet to be made on his teammate Ryan Jack.
"Ryan Jack is a doubt for tomorrow's game and we won't know on Ryan until later on today whether he is available or not," Gerrard was quoted by the club website.
"Zungu will be missing as well - we are hoping to have him back in a week to 10 days’ time - he's not got a big issue but unfortunately he's not available for us tomorrow, and the rest is as it was from the weekend."
Rangers are enjoying a perfect run of results across all competitions since the start of the season and they are aiming to extend their unbeaten streak to 22 games on Thursday.
"Winning games and going on unbeaten runs definitely breeds confidence and belief in the group," Gerrard added.
"Looking into tomorrow's game as well, I think the players have experienced playing against these players and this team not so long ago and we have shown that, certainly for big parts of the game that we could match Benfica and certainly compete.
"So this is a game that the players are very much looking forward to. We have major respect for Benfica individually and collectively.
"You are talking about high-level players and a high-level club, and a manager with big experience.
"We expect a very, very tough game tomorrow but a game that we are really looking forward to and are excited about and we couldn't be going into it in a better place if you think about the run of games we have been on."