O’Kane and Cooper limped out of United’s League Cup defeat to Liverpool on Tuesday with problems sustained in the first half at Anfield, adding their names to an absentee list which already included Liam Bridcutt, Pablo Hernandez and Pontus Jansson.
Bridcutt and Hernandez were ruled out of the trip to Merseyside with foot and hamstring injuries respectively while illness prevented centre-back Jansson from returning from a one-match ban.
United’s medical staff expect Jansson to be fit for Saturday’s Championship game against Aston Villa but Bridcutt and Hernandez will be missing again and Leeds are assessing the damage to O’Kane and Cooper ahead of the weekend.
Monk said: “I don’t think anything’s too serious and we can’t afford many injuries. We’ve got a thin squad as it is.
“(Jansson) was ill over the weekend and we’re very thin on the ground but whoever goes out on the pitch, I know they’re going to give spirit and quality.”
Leeds finished Tuesday’s quarter-final with right-back Luke Ayling in the centre of defence and the enforced changes early on left Monk with just one substitution to work with in the second half.
A fine performance from his players went unrewarded as Liverpool scored twice late on to reach the League Cup’s semi-finals and Monk said: “You never want injuries early. It limits your changes towards the end.
“But the pleasing bit is that we work on having to adjust. Sometimes players will have to play in positions they’re not used to. We had to chop and change and we had a makeshift back four at one stage but that’s part of the work we do. Everyone understands their roles wherever they have to play.”