On 14th December 2017 I wrote an article called Slumper Sinclair? gently deriding the notion Sinclair was in any kind of form slump. The numbers did not support the notion at that time. Talk about curse of the commentator!
Since the game after the article (against Heart of Midlothian – you probably remember it), Sinclair has completed only seven 90-minute matches. He only completed two in a row recently versus Rosenborg due to injuries to Dembele and Griffiths. With 3 goals since then, two against part-time Brechin City and relegated Partick Thistle, is the slump now truly on?
Sinclair has definitely lost his “first name on the team sheet” status. In his first season Sinclair started 43 of 50 matches. In early 17/18 he started 26 of the first 34. He has started only 16 of the last 31.

In his first season Sinclair scored 20 non-penalty goals but only 3 have followed since the 17th December 2017.

In that first season Sinclair underperformed against his Expected Goals (xG) which corrected itself in the early part of 2017/18 season.
Since then both his xG and actual output have fallen away. He is now underperforming significantly against his xG again suggesting an improved goal return is imminent.
But the more worrying trend is a downward one.
A slight glimmer of hope is that Sinclair has been creating slightly more chances (2.01 per 90m) lately but no near his initial season (2.3 per 90m).
However, his assist rate of 0.45 per 90m is the best of his Celtic career, outperforming his xA of 0.302 per 90m.

Also, the quality of the chances he is creating is increasing. In the 2016/17 season he averaged 0.126 xG per chance compared to 0.150 recently.
Has he adapted his game to be more of a provider?
Well, he is certainly taking less shots now. Averaging 3.41 per 90m in his first season that has now reduced to 2.35 per 90m. His shot accuracy is improving up from 41% to 52% recently. That is likely because he is making more shots from inside the box and less outside. Attempting 1.07 per 90m outside the box in 2016.17 it is now only 0.61 per 90m, almost half.
This suggests a great conservatism to his game: less risks being taken – more passes and less shots and more conservative shots.
The net result is that productivity in terms of Scoring Contribution is down.
Whereas in 2016/17 and early 2017/18 he was providing a scoring contribution every 119 minutes, it is now every 147 minutes. Previously he had 61 possessions between each scoring contribution, and now it is 71.
From a form perspective, what concerns me is if you look at the Packing data. This data is powerful as it recognises the contributions of both pass giver and receiver as well as the dribbling element.
As regards passing, he was bypassing opponents with 11% of his passes and that has dropped slightly to 10%.
More glaring is the drop off in dribbling. In his initial season he was successful with 1.73 dribbles per 90m. That figure has now fallen to 1.06 per 90m. He was previously successful with 50% of his take ons but this has now fallen to 36%. And he is trying less dribbles – previously 3.34 per 90m and now 2.9 per 90m. For a quick and wide layer like Sinclair this is a glaring reduction in attacking effectiveness.
Lack of confidence, under orders or diminishing pace?
Equally worrying is his movement off the ball as shown by Pack Receives. This measures a player’s ability to take in passes that bypass opponents, control and execute the next action. The number of Pack Passes Sinclair received has dropped from 9.2 to 7.48 per 90m since Dec 17. The Impect of that – indicating the number of players bypassed by value – drops from 47.43 per 90m to 34.15 per 90m.

Conclusion
I would call this a definite slump. Which came first, poor form or lack of playing time leading to lack of form momentum?
The symptoms are: more conservative shot selection, more passing (more assists but less shots and take ons), less dribbles and being less open to receive forward passes / diminished forward movement.
There seems to be a lack of confidence perhaps due to the player being aware of his own slight degradation of pace – this is speculation on my part from the evidence.
Sinclair can still be one of the most productive players at Celtic. But for over eight months now, performance levels are dropping.