The 24-year-old Irishman endured a season of frustration as an ankle injury kept him on the sidelines for four and a half months while his team-mates made a major impact in the Champions League.
But the former Falkirk, Sunderland and Hibernian frontman returned to fitness in time to score the winning goal in the William Hill Scottish Cup semi-final against Dundee United, while he put in a man-of-the-match display against his old Leith side in the final.
Boss Neil Lennon, however, faces losing the player after he turned down a new deal.
But the Hoops manager said: "We have already made him an offer - he has knocked that back, but negotiations will be ongoing.
"We really saw the best of him towards the end of the season. He looks physically in the best shape he has been in.
"He was such a pivotal player for us towards the end of the season, and his cup final performance was absolutely fantastic.
"The opportunity is there for him to go and make a good career for himself. He is very important to me."