The main obstacle to Percival would be that Saints simply wouldn't sell.
People are wrong to assume the Saints and Wigan top brass don't get on. They get on with each other just fine; it's possibly a better relationship than most other clubs have with each other.
But while I'm sure Wigan would love to have Percival in the centre (and as main goal-kicker) and make Gildart their explosive utility back, McManus must know there'd be complete uproar from the Saints supporters.
Unless Saints are in dire financial straits that we don't know about - as they were in the era that saw them sell Andy Platt and Gary Connolly to their deadliest rivals - I can't think of any conceivable deal that would see them send us their best prospect.