Celtic will take on Scottish Premiership rivals Aberdeen at Hampden Park in Glasgow in the final of the Scottish League Cup on Sunday afternoon.
According to The Scottish Sun, Sinclair is injured and will miss the clash against Aberdeen.
The former Aston Villa winger suffered a hamstring strain against Barcelona at Celtic Park in the Champions League on Wednesday evening. It has been reported that the 27-year-old has been ruled out for a month.
Former Celtic midfielder and manager Hay believes that Sinclair and the club will be disappointed with his absence.
Hay has also stated that he has been impressed with 24-year-old Celtic midfielder Stuart Armstrong.
“Scott Sinclair’s absence is disappointing for both the club and the player because he has made a vital contribution this season, with his pace a real asset to Celtic,” Hay wrote about Sinclair in The Glasgow Evening Times.
“He is clearly comfortable in that central berth – although I also thought he was outstanding as a right-back against Inverness a few weeks ago when he had to deputise in there because of injuries,” Hay wrote about Armstrong in The Glasgow Evening Times.
“But I have been impressed with how he has got himself back into the team – and what he has done on getting back in.”
Celtic are at the top of the Scottish Premiership table at the moment with 34 points from 12 matches.
Brendan Rodgers’s are eight points clear of second-placed Rangers at the top of the standings and have two games in hand. The Hoops are favourites to win against Aberdeen on Sunday.