The 20-year-old striker had a loan spell at Liege's rivals Lierse last season and scored eight goals for Stanley Menzo's team.
Watt had disagreements with Menzo, who questioned the Scotland under-21 internationalist's attitude, but his on-field ability attracted Liege, who are keen to complete the deal today. They have a Champions League qualification tie against Greek side Panathinaikos on Wednesday and want to register Watt for that match before tonight's deadline.
Watt rose to prominence in 2012 when he scored for Celtic in their 2-1 Champions League victory against Barcelona at Parkhead - just months after Neil Lennon had signed him from Airdrieonians for £80,000.
Lennon then sent him on loan to Belgium for regular top-team football but the player had hoped to reinvigorate his Celtic career under manager Ronny Deila.
Deila had wanted to sign Watt last season for Stromsgodset but could not complete the deal before the Norwegian transfer window closed.
On Saturday Watt played for the second-string Celtic side beaten 1-0 by German team St Pauli in a friendly.
Today the Glasgow club are to unveil new coach Haakon Lunov, Deila appointing the 31-year-old Norwegian for the second time, after taking him to Stromsgodset three years ago from Odds BK (then Odd Grenland).
Lunov worked with Everton and also gained experience with Liverpool and Bolton Wanderers, while graduating in England, before returning to Norway.
Deila said: "Haakon has an extremely high expertise in analysis, he is ambitious and very concerned with development.
"He is just the type of addition to the staff that we were looking for. He learned his knowledge by watching a lot of football at a high level. Haakon will teach and develop both the coaching staff and the players."
He has not played as a professional but earned degrees in science and football at John Moores University in Liverpool, and pedagogy and psychology at the University of Oslo.
Celtic travel to Poland this week for Wednesday's Champions League third-round qualifying tie first leg against Legia Warsaw, with Ivica Vrdoljak the latest Legia player to predict a win for his side.
Confidence in the Polish team is said to be high after their 3-1 weekend defeat of KS Cracovia.
Croatian midfielder Vrdoljak said: "We've managed two away wins in a row, but now we have a real challenge. We have a lot to prove when it comes to the Champions League. To qualify, we have to give everything and get a bit of luck too.
"But Celtic won't know about our fans. They are our 12th man.
"They can help us get the victory and if we can win in Warsaw, we can keep a clean sheet in Glasgow and qualify. We are a team that doesn't give a lot away. I think we have a great chance."