Janmaat reportedly damaged his hand by punching a dressing room wall in frustration, after Newcastle were beaten 3-1 by Southampton.
The Dutch full-back initially was expected to make a swift return, but scans revealed that he had broken a bone and he now is likely to spend the rest of the season on the sidelines.
Coloccini, meanwhile, has been suffering with a calf problem since February - and he is unlikely to make it back.
Even if the experienced Argentine had comeback to fitness it would have been doubtful that he would have been included by Benitez.
Coloccini has struggled this season, and his time on the sidelines has allowed Jamaal Lascelles to thrive.
Lascelles has been one of the Magpies’ top performers since Benitez took charge, and he is believed to have found his voice in the dressing room.
In better injury news, Benitez confirmed yesterday that both Kevin Mbabu and Yoan Gouffran were both nearly back to full fitness.