Sadlier facing three months out
The Lions received the news on Monday after a specialist looked at the 23-year-old's injury and the Dublin-born hit-man is expected not to train for about two months.
On the extent of Sadlier's hip problem, Millwall boss Mark McGhee told the club's official website: "The week before the season it looked like Richard was only just short of being ready to start against Rotherham.
"But it became clear that the hip which was operated on in the summer has not fully healed and Sads (Sadlier) has been back to see the specialist who performed the operation.
"We were told at the time that this problem can take anything from 12 weeks to almost a year to clear up, and we were hopeful that Richard would be fit in just over the 12 weeks.
"But now we are resigned to the fact that he will not kick a ball again for two months."
With Sadlier's team-mate Steven Reid out for a month an ankle problem and Joe Dolan also out long term, McGhee will need to bolster his squad after a slow start to the new season.
McGhee added: "I'm working overtime at the moment looking at ways of bringing players in whilst working within the parameters that have been set.
"With the current financial situation we cannot go out and buy replacements so we have to work with what we've got and look for loans or the sort of deal that we have done to get Andy Roberts in."
Meanwhile, Millwall have signed young Irish goalkeeper Terry Masterson to a three-year trainee scholarship with the South London club.
Masterson has played for the Republic of Ireland at the Under-16 level and looks to be another future prospect for The Lions in the years to come.