Jonjo Shelvey went on loan to Blackpool last season & scored 6 goals in 10 games. It's fair to say that had he stayed for longer it's quite possible he'd have ended up with the POTY award. Doesn't mean he's going to make the grade at the top level though - If this lad is so good, why hasn't Fergie shown any interest. After all, he'd only cost a few million more than what you paid for the young lad from Crewe, so it wouldn't put too much dent in United's transfer budget. It would also give you strength in depth in an area that most United fans feel is lacking.
It's one thing doing well for a lesser side, but it doesn't always mean a player will replicate it when playing for a big club. ironically enough, Robbie Keane is a prime example. Great player for Spurs when they were regular mid-table finishers. But he struggled big-time when he moved to Anfield in a side that was ready to challenge for the title. He was never given the time & space he had when he was at Tottenham because sides knew they had to work that little bit harder. Keane himself didn't have the extra quality needed to make the difference. Only time will tell if Sigurdsson has what it takes to make, what is, a big step up.