Mauricio Pochettino has confirmed that Dele Alli suffered a hamstring injury during Tottenham's 2-1 victory at Fulham on Sunday.
The England international had to be substituted in the closing stages at Craven Cottage but Pochettino does not know how long he will be sidelined for.
Alli's absence is the latest blow to Pochettino who was without Harry Kane, Son Heung-min, Lucas Moura, Mo Sissoko and Victor Wanyama for the London derby.
Fulham led at half-time through a Fernando Llorente own goal but Alli equalised for Spurs before Harry Winks grabbed a last-gasp winner for the visitors.
Pochettino said of Alli's injury: "We all see the reaction, he suffered an injury in his left hamstring.
"We need to assess him in the next few days. We hope it is not a big issue. But first of all, we need to assess.
"Some injuries appear in a period with a more busy schedule and another opportunity opens the door for players not consistently in the starting XI or on the bench. It is great opportunity for different players to show quality.
"Sometimes in football, it is not tactics or formation – you have to believe. It was an unbelievable finish. That's why it is so important to believe."
Spurs, 2/9 to finish in the top four this season take on Chelsea in the semi-final second leg of their Carabao Cup semi-final on Thursday. Pochettino's team are 4/5 to qualify for the final and 3/1 to win the competition outright.