Aberdeen: Derek McInnes looks to add a defender in January
Aberdeen manager Derek McInnes intends to use the January transfer window to increase his defensive options.
"Whether that can be done at the start of January, or as we go through it, remains to be seen," said McInnes.
"But we're confident we can bring in the necessary players that can help us meet the demands that's put on us."
McInnes's Dons side sit in third place in the Scottish Premiership going into a busy period of festive fixtures, starting with the visit of Motherwell on Boxing Day.
Scotland under-21 international Robertson damaged knee cartilage in Aberdeen's League Cup quarter-final victory at Motherwell in October.
A calf strain meant Considine missed Saturday's 4-3 victory against Inverness Caledonian Thistle, during which Reynolds was taken off with a facial injury.
English centre-back Hector moved to Pittodrie in July on loan from Reading and that six-month deal ends in the New Year.
McInnes added: "Clark Robertson won't be fit until February, Mark Reynolds will be fit for next week, and we are still waiting on scan results coming back for Andy Considine.
"But he will probably be out for a few games anyway so we are a little light defensively.
"Michael Hector will go back to Reading after the Kilmarnock game on the fifth [of January] so I do believe we will have to strengthen defensively in January.
Meanwhile, Aberdeen midfielder Peter Pawlett will make his 100th appearance for the Dons if he features against Motherwell.
The 22-year-old has impressed during this campaign, scoring four goals and becoming a mainstay in McInnes's side following a loan spell with St Johnstone last season.