Millwall will not be charged by the Football Association over the crowd trouble during their 3-2 defeat by Middlesbrough at The Den in February.
Play was halted when missiles were thrown after Boro went 2-1 up.
Lions chief executive Andy Ambler said: "The FA have informed us that they will monitor every game between now and the end of the season.
"They have advised us that they can take retrospective disciplinary action should there be any further incidents."
A linesman and visiting goalkeeper Jason Steele were the subject of objects thrown from the crowd after Scott McDonald gave Boro the lead.
Referee Chris Sarginson delayed proceedings for several minutes while he spoke with Millwall boss Kenny Jackett and opposite number Tony Mowbray.
"The FA have taken into account the arrests, and the steps Millwall FC have [implemented], and continue to implement, to stop similar incidents occuring in the future," added Ambler.
"We pride ourselves in the passion of our support and on Tuesday night we saw that with a fantastic atmosphere at The Den against QPR.
"That is the type of support we want to help the club continue to move forward.
"Incidents such as those that blighted the Middlesbrough game obviously hinder the progress we are making."