Only last week, manager Ronny Deila spoke about the 20 year-old hitman having something special he could bring to the team.
But, despite having responded to Deila's demands he gets his fitness up to the required level, Watt was left on the bench for the qualifier against KR Reykjavik.
He did, however, play the entire 90 minutes of Saturday's friendly against St Pauli in Germany, but failed to make any real impact.
Watt was later criticised by Deila for trying to claim the ball when the Hoops were awarded a late penalty, causing Bahrudin Atajic - who had been assigned spot-kick duties - to lose concentration before sending his shot over the bar.
The £80,000 signing from Airdrie burst onto the scene in 2012 when, as an 18 year-old, he scored the winning goal against Barcelona in a Champions League match at Parkhead.
He was handed an improved contract soon after. But, his career stalled and then-manager, Neil Lennon, rarely lost patience before sending him out on loan for all of last season to Lierse in Belgium.
The plan was to try and force Watt to mature. But, after making a good start, he fell out with Lierse coach, Stanley Menzo, who openly criticised his condition and attitude and sent him to train with the reserves.
Returning to Parkhead this summer, where a new boss was no in situ, was expected to offer Watt a fresh start.
Instead, he is once again heading to Belgium, this time with his Celtic career behind him.
Standard Liege first contacted the Hoops about his availability in January, but a deal could not be agreed.
Having recently had another offer again rejected, Standard Liege have now met Celtic's valuation, and hope to complete the transfer in time to register him for Wednesday's Champions League qualifier against Panathinaikos.