The 22-year-old has missed United's last three pre-season friendlies with an ankle injury and manager Greg Abbott concedes the Dubliner's fitness is becoming a worry.
Should the Republic of Ireland Under 21 hit-man face an extended spell on the sidelines it will be his second consecutive pre-season absence after a broken foot ruled him out in 2011.
"We are going to have a look at Paddy's ankle - he has got a slight problem with it," Abbott told the Carlisle News and Star.
"Dolly [physio Neil Dalton] will assess it and then decide where we go from there."
Abbott has also been forced into resting new signing Danny Cadamarteri for several days as the former Everton striker looks to recover from a hamstring niggle.