After making a brief cameo off the bench in the first leg last week, Hazard played 72 minutes at Valdebebas and could now face his former club from the start in the pivotal return game at Stamford Bridge.
Real Madrid have it all to do after drawing the opener 1-1 and they are playing catch-up in La Liga too, after Atletico Madrid had earlier scraped to a 1-0 victory against struggling Elche.
Elche's Fidel Chaves missed a 91st-minute penalty in what could prove a pivotal moment in Spain's thrilling title race, with Atletico maintaining their two-point lead over Real Madrid and stretching the advantage to five over Barcelona. Barca travel to Valencia on Sunday while Sevilla, who are now six points back, play their game in hand on Monday against Athletic Bilbao.
Atletico's win meant victory for Real Madrid was essential and they had to work hard for it against a determined Osasuna side, Eder Militao breaking the deadlock in the 76th minute before Casemiro made it two four minutes later.
Zidane's squad has been stretched by absentees in recent weeks and he left Luka Modric and Toni Kroos on the bench, while handing Hazard his first start since January 30. There could be a fresh concern, though, after defender Raphael Varane was taken off at half-time.
Hazard was bright from the start, even if his precision was missing. He drove through midfield but skewed a shot wide. The best chance fell to Hazard from a Marcelo cross but he failed to catch the volley cleanly, instead scuffing it into the ground and goalkeeper Sergio Herrera did brilliantly to save.
Karim Benzema had already gone close and Militao had a pair of headers early in the second half that again drew Herrera into action. Hazard faded and departed with 18 minutes left. Osasuna were tiring too and finally Madrid got the breakthrough, Militao making it third time lucky, rising high to head in Isco's corner.
Four minutes later it was two, Benzema driving in from the left and slipping a reverse pass through for Casemiro, who stretched to control and instead poked the ball directly into the net.