Dembele has been out of action since rupturing the "tendon in his femoral bicep in his left thigh" during the Catalans' 2-1 victory over Getafe on 16 September.
At the time, Barcelona announced that the France starlet was going to be out of action for three-and-a-half months after undergoing an operation carried out by specialist Sakari Orava in Finland.
The 20-year-old winger was therefore tipped to be unavailable for the rest of the year, missing around 20 games including the highly-anticipated visit to Real Madrid.
However, earlier this month Dr Orava suggested that Dembele could return slightly ahead of schedule and in time to play some minutes in El Clasico.
Over the weekend, Dembele himself gave Barcelona fans fresh hope he will be available to face Real after revealing that he plans to be back in full training in only two weeks.
But Valverde has claimed that he won't take any risks with Dembele after pointing out that El Clasico will come just before a two-week winter break which could help the forward regain some fitness before returning to action next year.
Asked in a press conference whether he expects Dembele to play against Real on 23 December, Valverde said: "The word impossible does not exist but it's difficult."
"It has been a serious injury, he has been out for a while and, if we have to wait another week or two, we will do it. We cannot take any risk, even more after having been without him for so long. It's good that he is eager to return but it's better to restrain a player than to push him. I must be cautious."
Elsewhere, Valverde also provided positive updates on the recoveries of Sergi Roberto and Andre Gomes ahead of the second leg of the Copa del Rey last 32 clash against Murcia on Wednesday night (29 November).
The two midfielders have been out of action since suffering respective hamstring injuries during the Champions League draw against Olympiakos on 31 October.
However, they returned to training last week and Valverde claims that both could be back either against Murcia on Wednesday night or when Barcelona host Celta Vigo in La Liga on Saturday.
"Regarding tomorrow's game [against Murcia] we have to wait. Yesterday [on Monday] they [Gomes and Roberto] completed a big part of the training session. On Saturday they also worked with the team. I don't know if for tomorrow [they will be available]. For Saturday we are optimistic but we have to wait. We think that soon they will be playing again."
Meanwhile, Arda Turan is yet to play a game this season having been plagued by different mysterious injuries.
Last week, Barcelona revealed that the Turkey international missed preparations for the clash with Valencia due to a dental abscess.
Some in Spain have questioned his recurrent fitness issues amid reports linking him with a January move away from the Nou Camp.
But Valverde said: "I am the manager and I absolutely trust the players and the medical team. If the player has a problem in the ankle, he has an ankle problem, I don't think about anything else."
The manager also confirmed that he will give some youngsters a chance to prove themselves against the Second Division B team with Barcelona holding a 3-0 advantage from the reverse fixture.
Lionel Messi, Luis Suarez, Andres Iniesta, Sergio Busquets, Jordi Alba and Samuel Umtiti will miss the game having been given two days off to recharge the batteries ahead of Saturday's clash with Celta.