I think it was purely a financial decision. Fox has shown he is a decent enough LB (not superb by any stretch). Whilst Celtic have not made a significant profit but they gained £1.8m in cash which was needed to help fund the recent transfers/loan signings. Mowbray has not been quiet in January you may have noticed! The decision might have been made easier for Mowbray if Fox hadnt 'settled' but I dont know if thats the case.
Mowbrays January signings
Loan
Keane (Though this is rumoured to have been funded by Dermot Desmond)
Kamara
Braafheid
Signed
Paul Slane
Rasmussen
Rogne
Hooieveld
Obviously St-Ledger never arrived and the deal for Ramis fell through late on, apparently his personal terms increased dramatically late in the day and Celtic walked away.
Good news for O'Dea.
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