Ibrox boss Giovanni van Bronckhorst has entered his first window since taking the reins in November, while opposite number Ange Postecoglou is hoping for a less frantic period after his extensive summer re-build at Parkhead.
Squad depth is vitally important for both Old Firm clubs ahead of the Scottish Premiership title run-in, given they are competing in not just the league, but in European competition as well.
The top-flight in Scotland resumes on Monday after the winter break when the Hoops face fifth-placed Hibernian before Rangers take on Aberdeen at Pittodrie 24 hours later.
If either manager opts to complete further business in the transfer market, then new faces will need to be assigned a shirt number.
With that in mind, GlasgowWorld takes a look at which numbers are available for any potential new signings.
Which squad numbers are available?
Rangers manager Van Bronckhorst has deployed his favoured 4-3-3 formation since taking over from Steven Gerrard in November. The current Light Blues squad does not require wholesale changes this month, but the Dutchman is expected to add defensive reinforcements.
Van Bronckhorst’s biggest worry will be uncertainty surrounding the long-term future of Connor Goldson, who has been a mainstay in the side, and fellow centre-back Leon Balogun who is also out of contract in the summer.
Both players are deserving of new contracts but it appears so far that little progress has been made to extend their stay at Ibrox beyond the end of the season.
Defensive midfielder James Sands became the club’s first January addition on an 18-month loan deal from New York City and he snapped up the number 19 jersey vacated by Croatian Nikola Katic, who is currently on loan to Hajduk Split in his homeland.
Several numbers are available for any potential signings, including 12 and 16 - the latter was assigned to Nathan Patterson last season, but the Scotland international has since departed for English Premier League side Everton.
Intriguingly for Rangers fans, the number 9 shirt has become available following Jermain Defoe’s decision to call time on his three-year stay in Glasgow yesterday.
Meanwhile, Celtic boss Postecoglou has been renowned for his high-pressing style of play and the Australian has already completed the signings of four new players.
Attacking midfielder Riley McGree looked set to make that five but talks have broke down in the last 24 hours, with the 23-year-old now expected to join Middlesbrough.
Japanese trio Yosuke Ideguchi, Daizen Maeda and Reo Hatate have all selected their numbers - 21, 38 and 41 respectively, while exciting youngster Johnny Kenny signed a five-year-deal and took the number 24 shirt.
Postecoglou is working hard to secure permanent deals for Portuguese winger Jota and centre-back Cameron Carter-Vickers, who are both currently on loan from Benfica and Tottenham respectively.
Other options available for new additions are; 22, 25, 27, 31.
If, like some players in Europe’s other top leagues, a new signing wants a higher squad number than 40, then there are plenty of options to select from for both clubs.
Which squad numbers might become available?
Rangers defender Connor Goldson and playmaker Ianis Hagi have been linked with moves, numbers 6 and 7 could become available, while the Light Blues have reportedly turned down bids for Borna Barisic, who wear 31.
Out-of-favour Jack Simpson is almost certain to depart on loan in January, so the number 15 shirt is likely to be freed up, with fellow outcasts Brandon Barker (21) and Jake Hastie (32) also expected to leave.
As for Celtic, rumours have circulated that goalkeeper Vasilis Barkas, defender Osaze Urhoghide and midfielder duo Ismaila Soro and Boli Bolingoli could all potentially depart this month, meaning the numbers 1, 12, 23 and 26 could become available.
Reports suggest winger Mikey Johnston could also exit on loan but Postecoglou might be reluctant to let the youngster go in this window, given the number of injuries picked up to key attacking players pre-Christmas.