Forrest has been an integral for the Celts this season.
He has scored 14 goals and provided 9 assists so far, making him the second highest assist provider, behind James Tavernier.
The 27-year-old also scored the deciding goal in Old Firm derby against Rangers.
In the game against Aberdeen, the Scotsman scored a stunning goal, which will certainly be a contender for the best goal of the season.
Now, the former Hoops player Hay said,
For me, he has been Celtic’s best player for the past 18 months. It’s between James and Callum McGregor for Player of the Year and I think James is just ahead.
He also praised Forrest’s performances in the big matches.
Hay believes that the winger’s hardworking and calm nature has reaped rewards.
Hay also praised Tom Rogic’s performance, who scored a goal in a 3-0 win against the Dons in the Scottish Cup semi-finals.
He also added that Neil Lennon is backing his players a lot, which is making them an unstoppable force.
Lennon, the makeshift manager, has inspired the Bhoys to six wins out of eight matches, since taking the helm, midway through the season.
The former Hibernian manager looks a favourite for the full-time manager job after notching up a crucial win in the Derby and in the Scottish cup semis.
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The Hoops are also hopeful of winning a domestic treble this season.