In the ninth minute Mount went in for a tackle on Valencia midfielder Francis Coquelin, and the Frenchman left something extra in the challenge, leaving Mount to hobble away.
Mount initially looked to have recovered from the challenge as he played on but in the 16th minute he was no longer able to continue.
Pedro would enter the game at Stamford Bridge in place of the 20-year-old England international.
Should Mount be forced to miss time it would be a blow for Chelsea, with the former Derby County loanee having started well in his first season with the Blues senior team.
The Chelsea academy product has scored three goals in five Premier League appearances this season, with his form earning him his first Champions League start on Tuesday.
Mount also made his senior England debut in the recently completed international break, with the midfielder coming in off the bench against Bulgaria and Kosovo for his first two caps.
The midfielder has become one of several Chelsea academy products to shine in the early season as the club turns to youth under new manager Frank Lampard.
The shift has partially been down to the club's two-window transfer ban but in any case, several players aged 21 or younger are stepping up for the Blues.
Mount, Tammy Abraham and Fikayo Tomori have impressed under Lampard, with all three of the academy products having spent last season on loan in the Championship.
Abraham is tied for first along with Sergio Aguero in the league's Golden Boot standings, having scored seven goals thus far.
The youngsters have even earned comparisons to Manchester United’s fabled Class of '92, as Chelsea have recovered with two wins and two draws since a 4-0 opening-day loss at Manchester United.
Chelsea return to action with a major challenge at the weekend, as defending European champions Liverpool visit Stamford Bridge on Sunday.
The Blues will then begin their Carabao Cup campaign next Wednesday as they take on League Two side Grimsby Town.