Pompey seal loan return for defender
Michael Appleton finally got the go-ahead by chief executive David Lampitt to bring the West Brom defender back to Fratton Park.
However, the left-back will not arrive until next week.
Mattock is set to be included in the Baggies squad for their FA Cup clash with Cardiff this weekend.
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Hide AdThe youngster arrived at Pompey for an initial loan spell which came to an end after the 2-0 win at Watford on Monday.
Blues boss Appleton was then forced to wait on whether Mattock could return to Pompey as the club sought clarification from the Football League as to what transfer deals they could make in the current window.
The former England under-21 international will now come back to the south coast, although it is unclear whether Pompey are now able to make further moves in the transfer market.