The winger returned early from international duty with Northern Ireland after suffering the injury ahead of the World Cup qualifier against Germany and sat out Saturday's game at Derby as a result.
Monk has now confirmed that Dallas will miss Leeds' clash against Wigan on Tuesday after undergoing a second scan to determine the extent of the injury but refused to put a timescale on when the 25-year-old would be fit again.
“We’ve rescanned him and it’s likely that he’s going to be out for a little bit,” Monk said. “How long that’ll be, we’ll probably know in the middle of this week but it doesn’t look like he’ll be available for Tuesday. Whether it’ll be the weekend we’ll see.
“We won’t really know until midweek exactly what sort of length it’s going to be.”