Sinclair enjoyed a hugely successful season with Celtic in 2016-17 and was one of the best players for the Hoops.
The former Chelsea and Manchester City winger, though, has not been at his best so far this season.
The performances of the 28-year-old have been disappointing, and it is clear that he needs to step up his game.
Former Rangers striker Boyd, who is now on the books of Kilmarnock, has pointed out that Sinclair has been poor.
The 34-year-old Scotsman, though, has praised Sinclair for his mentality to take the penalty against Motherwell on Wednesday evening.
With the Hoops trailing 1-0 at Fir Park, the former Swansea City and Aston Villa winger stepped up to take the penalty in the 88th minute and scored to rescue a point for Brendan Rodgers’s side.
Boyd wrote in The Scottish Sun: “Look at Sinclair. He’s been poor this season, with his performances for Celtic nowhere near as good as they were last season.
“But what did he do when Celtic got that late penalty at Fir Park? He grabbed the ball. Watching it back, there was no chance he was going to miss. He smashed it into the roof of the net like he was in a training game.
“He was under huge pressure after what had happened at Hampden. Had he missed, the team’s unbeaten run would have ended and his excellent performances would have been quickly forgotten.
“But he didn’t miss, did he? No, he scored when all eyes were on him because he’s got the mentality to play for a huge club like Celtic.”