The Madrid giants advanced thanks to their 3-0 first-leg victory, but after confirming he had finally penned a new deal to 2020 agreed back in September, Zidane hinted he feels his job could be under threat, per AS:
"I just go game to game, year to year. I cannot think two or three years down the line, as I know how it is here, even having a contract, which means nothing. It is not easy for the players who play less. We played a good game. We maybe lacked a bit of gas at the end."
Zidane, 45, has won La Liga, two UEFA Champions Leagues, two FIFA Club World Cups, one Spanish Super Cup and two UEFA Super Cups since taking the top job at Real in January 2016.
However, 2017-18 has been difficult for the Frenchman, as his team have performed relatively poorly.
Their chances of successfully defending the league title are effectively gone, as they sit fourth in La Liga, 16 points behind leaders Barcelona.
They are through to the quarter-finals of the Copa del Rey, but Wednesday's draw was Real's third winless game in their past four.
Zidane made 10 changes from Real's draw with Celta Vigo on Sunday for the Numancia clash, and Los Blancos were twice ahead at the Santiago Bernabeu through goals from Lucas Vazquez.
Guillermo Fernandez equalised twice—first on the stroke of half-time and then in the 82nd minute—exposing defensive frailties that have dogged Real all campaign.
Given his remarkable record as Real manager, it is unlikely Zidane is in any imminent danger of being sacked.
However, Real face a potentially season-defining tie with Paris Saint-Germain in the Champions League round of 16, with the first leg set to take place on Feb. 14.
Should Real fall to the Ligue 1 leaders and thus fail to reach the quarter-finals of the Champions League, it could spell the end for Zidane.
He appears to be aware that, despite his successes, he has little job security at the Bernabeu. As such, Zidane will be desperate for his side to get back to winning ways on Saturday, when they face Villarreal in La Liga.