The former Derby and Nottingham Forest star was shortlisted alongside Kilmarnock striker Kris Boyd and team-mates Virgil van Dijk and Fraser Forster.
And Commons, who scored 28 goals in all competitions, was convinced the pair would contest the honour after playing such a major role in the club's stroll to a third successive title.
"It's great to be nominated by your fellow professionals but it is a bit overwhelming really," he said.
"I really thought it was going to be Virgil or Fraser who can both hold their heads up high this year as they have been outstanding for us.
"I think Fraser needed recognition for the quality season he has because some of his saves have been simply outstanding and breaking the clean-sheet record showed how good a goalkeeper he is.
"Then when you look at Virgil, considering he is only 22 and this is his first season at Celtic. Coming to a new country to a new team is always difficult but the way he has hit the ground running has been amazing and I have no doubt he is capable of playing at the highest level.
"So I was truly gobsmacked when I got the shout although I think when you are scoring on a regular basis you are always in the limelight.
"People are always taking note of what you are doing and luckily for me I have been given the opportunity to play a bit more advanced in games this season.
"But I think my award and the nominations reflect well on everyone at Celtic, particularly the manager and his backroom staff.
"They have brought in a very high calibre of player and they deserve plenty of credit for it but we'll be looking to kick on once again next season and I'm sure they're all hard at work looking for new additions that will make us better."