And Chelsea boss Antonio Conte was far from reassuring when addressing Morata's status on Friday.
"I don't know [when he'll be back] because he has this problem and we tried a lot to find a solution to solve the pain in his back," Conte said.
"If you ask if me he needs one month, one day, two months, the rest of the season, I don't know.
"For this reason I'm a bit worried because you know very well the importance of the player."
Goal can reveal that despite a number of tests on Morata, the exact source of the pain in the striker's lower back has yet to be determined.
The 25-year-old has been working hard to rehab his injury in a regimen that includes exercises in the swimming pool, but the pain still remains.
That match is just four days before the first leg of Chelsea's Champions League round-of-16 tie against Barcelona – a match in which Morata is desperate to feature.
Despite the mysterious nature of Morata's injury, there is currently no concern that the striker could be in danger of missing Spain's World Cup campaign this summer.