The outstanding Irish flanker has failed to fully recover from a broken bone in his hand ahead of the Heineken Cup clash at Northampton.
But Easterby says it will take more than that to keep him out of Llanelli's make or break showdown with the Saints.
And he has vowed to play through the pain in order to get the Scarlets through to the quarter-finals.
"The specialist said he is happy that the break will not cause any long-term problems as long as I was happy to deal with the pain," said Easterby.
"I was given a choice - but this is a massive game for us and there was no way I was going to miss it because of this injury.
"It's still painful and ideally I would rest it but hopefully on the day I will forget about it when the adrenaline starts pumping. My main worry is just not letting anyone down.
"Obviously I have aspirations to play for Ireland in the Six Nations and that is in the back of my mind. But the Irish management said they were happy to trust my judgement.
"Hopefully my decision will be proved right and we will win through. I would keep my fingers crossed - but unfortunately I can't."
Easterby's decision to play will be music to the ears of coach Gareth Jenkins who was facing an injury crisis at the start of the week.
Now he not only has his first choice openside available but also centre Mark Taylor, props Iestyn Thomas and John Davies and talisman Scott Quinnell. However Wales pair Mark Jones and Leigh Davies have failed to recover in time.
"Two years ago we would not have gone to Northampton with much confidence after losing Mark and Leigh, but that's not the case now with the new strength in depth that we have here," said Jenkins.
"But it is a big boost for us to have the others fit, though if it was any other game we probably would not ask Simon to play but this Sunday is so important.
"The injuries have badly disrupted our preparations but now we just have to give everything we have to winning at Northampton."
Scarlets: B Davies; G Evans, M Taylor, M Watkins, S Finau; S Jones, D Peel; I Thomas, R McBryde, J Davies, V Cooper (capt), A Jones, D Jones, S Quinnell, S Easterby. Replacements: P John, A Gravelle, C Wyatt, D Hodges, M Phillips, G Bowen, T Selley.