Allardyce knows that he has signed a good player in ban Heerden, but he feels that he needs to toughen up to cope with the demands of the Premier League.
The 25 year old has just landed a two year deal with Rovers, arriving on a free transfer from Brugge.
Allardyce told the club's official website: "We need to build him up as he is not the biggest of lads, then I am sure he will make an impact as the season progresses.
"We have signed a very talented young man. He is a great addition to the squad. He has lots of pace, lots of ability and can play wide right and left.
"He's had a couple of injuries, which have been hiding his talents but we have monitored him over the last few months."