Magennis has arrived at Blundell Park to pep up a forward line which is currently short of the services of Adam Proudlock and Nathan Jarman due to injury, and Barry Conlon through suspension.
Proudlock is sidelined after suffering ankle ligament damage in the home defeat to Burton Albion last weekend, while Jarman broke a bone in his foot during training on Monday. Both players are expected to be out for around a month.
Conlon serves the third game of a four-match ban, while midfielder Danny Boshell sits out the final game of his two-match suspension.
The Mariners have won just once at home this season and another defeat could see them finish the weekend inside the bottom two of the table.