The Welsh winger hurt his hamstring midway through the second half of Sunday's Scottish Cup semi-final win over Aberdeen and has now been ruled out both tonight's match at Kilmarnock and the vital Easter Sunday meeting at Ibrox which could decide this season's Scottish Premier League title.
The 25-year old said he was disappointed t be sidelined for the crunch match but is confident his teammates can do the job.
"I'm gutted the injury has ruled me out against Rangers," he said.
"I was desperate to play in the game, but it's not worked out that way.
"We've got a good squad so I'm sure the lads will cope, but it is hard to accept I'm missing it."