Thomas Rooney - cousin of Manchester United and England star Wayne - signed non-contract terms with the club this week and will go into the Champions League squad and could be set for a place against European Champions Liverpool.
He may also get a run-out in Saturday's home pre-season friendly with Irish Premier League champions Glentoran (kick-off 6.00pm).
Rooney, 20, has previous experience with Tranmere Rovers and Macclesfield Town but was released from the Moss Rose at the end of last season and has been training with the Saints for the past two weeks.