And there are doubts over whether Ball and Thomas will make it back for the much-anticipated Round 2 clash with Carlton and their former coach Mick Malthouse.
Ball is still on the comeback trail from major knee surgery, while Thomas is yet to return from an off-season ankle operation.
Didak is yet to play this pre-season because of a hamstring strain.
In an injury update on their website on Wednesday, the Magpies said Ball and Thomas were both three to four weeks away from returning.
The Magpies open their season against North Melbourne on March 31.
Their clash with the Blues, whose new coach Malthouse led Collingwood to the 2010 flag, is less than four weeks away.
Didak, 30, who played just 11 games in an injury-riddled AFL season last year, is forecast to return in three weeks.
"Alan is getting towards the sharp end of his recovery," Magpies football manager Geoff Walsh said.
"He is stepping up his training and has had no setbacks."
Key defender Ben Reid, who injured his knee against West Coast in early March, is expected to recover in time for Collingwood's season-opener.
Versatile big man Lachy Keeffe, who suffered a major knee injury in round nine last year, won't be back until well into the home and away season.