Off topic but longer than De Gea?
Signed the same summer, not exactly sure who was first.
De Gea was confirmed on 29 June 2011.
Jones was announced earlier, on 13 June but he was away with England U-21 at the same tournament De Gea was at with Spain U-21. Jones was said to be signing after the tournament but I don't know the exact date he penned the contract. England were eliminated before Spain won it so it would have given Jones a few extra days to actually sign but I don't know if he did. All that's from Wiki.
Anyway, I'm having Lingard who also turned pro in the summer of 2011 on some unknown date. Technically he could have played for us before turning pro, perhaps in emergency circumstances so he was serving a purpose I suppose. Harvey Elliott, now at Liverpool made his Fulham debut at 15 so Lingard was available to do the same for us if we really needed him to.
Not sure I'd normally consider being with the club as a schoolboy as time-served, but seeing as it's about a 3-way tie with those 3 players as to who has been here the longest as a professional footballer I think it's a reasonable way of splitting them. If we went by actual first team debut, which wouldn't be unreasonable either Lingard would of course have been here the shortest of him Jones and De Gea.