The striker was with United on Wednesday to monitor his knee. However, it later became swollen after a training session and he will now require a scan nextweek to discover the full extent of the damage.
On target: Welbeck after scoring in the Carling Cup earlier in the season
Preston manager Darren Ferguson was downbeat about Welbeck's chances of playing for them again.
He said: 'The way it is looking at the moment, he will go back there and thatwill be it. Danny is going for another scan on Monday but it's not lookinggood.'
It is feared the England Under 21 international will now need an operation on hisknee with a torn cartilage the likely diagnosis.
However Ferguson,son of United manager Sir Alex Ferguson, has not ruled out the possibility of a return before the end of the season.
Point in the right direction: Welbeck is one of Uniteds most promising youngsters
He added: 'On Wednesday night I got a phone call from the physio. I didn't really want to answer it and I could tell by the tone of the physio's voice that it wasn't good news.
'We wish the lad all the best because we don't want this to turn out to be a serious injury. If the scan on Monday shows no operation is needed, there is the possibility that Danny might come back, but I'm not overly optimistic.'
Welbeck has played seven times for Preston, scoring two goals including a goal on his home debut in a 2-0 win over Ipswich.
Welbeck seals loan move to Preston as Ferguson Jnr raids Manchester UnitedPreston hope to complete loan signing of Manchester United striker Welbeck