Dylan Reid has been a long-time target for the Scottish champions.
The 17-year-old turned down a move to Celtic earlier in July having been given permission to speak to the club following an agreement over a six-figure fee.
"We are pleased that Dylan will remain at St Mirren and look forward to him continuing his development at the club," the Buddies said at the time.
However, according to the Scottish Sun, the move is back on.
Further discussions have been held and a deal could be completed in the coming days with the teenager joining up with Celtic’s B side which are currently plying their trade in the Lowland League.
Reid, a highly-rated Scotland youth international, featured for St Mirren in the Premier Sports Cup but has not played in any of the five league games and wasn’t in the squad for the weekend’s win over Hibs.
Meanwhile, Malick Thiaw has completed a move to Serie A champions AC Milan.
The centre-back was strongly linked with a move to Celtic earlier in the summer prior to the arrival of Mortiz Jenz.
Thiaw has left German giants Schalke 04 for Milan who are preparing for another Champions League campaign. The 21-year-old featured in three Bundesliga matches this season, his latest a 6-1 defeat to Union Berlin.
He has signed a five-year deal with the San Siro club and becomes Stefano Pioli’s fifth summer signing.