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The return of Ross McCormack and Warren Feeney from injury meant that Magennis fell down the pecking order at Cardiff and he was allowed to join League Two side Grimsby Town on a one-month loan deal on 15 October 2009 He made his debut against Rochdale on 17 October 2009, which was to be the last game managed by Mike Newell, who was sacked the following morning. Magennis made just one more appearance for the side before, on 29 October 2009, he was told that he and fellow loanee Arnaud Mendys loan contracts were to be terminated ahead of schedule. Caretaker manager Neil Woods commented that "As the caretaker manager I think I need to be fair to the players that we've got here first. They've been a little bit of a victim of the circumstances of the last two weeks. They are not going to get in the eighteen and I don't think it is fair to keep loan players here."