England prop Kieran Brookes is a major doubt for the Six Nations after undergoing surgery on his left knee.
Brookes won the first of his 15 international caps in 2014 against New Zealand, but he may have to wait to add to that tally after suffering a cartilage injury last week.
As a result Brookes did not feature in Northampton's 25-23 defeat to London Irish on Saturday - the latter's first points of the premiership season - and Mallinder confirmed he is likely to be missing for "quite a few" weeks.
With the Six Nations due to begin on February 6, the 25-year-old may struggle to fit in time to make Eddie Jones first squad as England coach.
"Kieran Brookes has an operation on his knee a couple of days ago and he's going to be out for quite a few weeks," said Mallinder after Saturday's loss.
"It's a big blow.
"He was someone who was playing really well and our scrum was going really well, so it's a blow."
Brookes made three appearances for England at the Rugby World Cup - all from the bench - but was unable to stop the host nation going out in the pool stage.