The Terriers are preparing to host Birmingham City in a league fixture tomorrow night but will be without Vallejo, who has not played since being substituted in the second half of the 3-2 win over Blackburn Rovers at the end of September.
Head coach Carlos Corberan expects the Spaniard, who scored in the victory over Blackburn, to miss approximately another two weeks of action.
"Alex Vallejo has taken half a step back," said Corberan.
"The reaction of his knee was not what we hoped so the medical staff have changed their treatment plan and we think he should be back in around two weeks.
"I have no doubt how important he can be in the team but unfortunately he's picked up that injury."
The Terriers did not pick up any fresh injury concerns following their 2-0 win over Hull City on Saturday.
Matty Pearson was forced off due to a stomach virus but that is not expected to have an impact on his availability for tomorrow's visit of Birmingham.
Corberan added: "I expect to have all the same players available as the last game.
"Matty Pearson went off with a stomach virus but he's been back in training and he's fine."