Van der Vaart was obviously a bigger name at the time, Eriksen also joined Spurs that time so it was a hard competition for him then. I guess in your opinion it is a mistake from United to buy Lukaku. He as you say flopped at Chelsea..
Didn't really flop, though, did he? He left as he wanted games. Very similar to Pogba.
Sigurdsson, on the other hand, played, but wasn't at the required standard. A bit like Moussa Sissoko. Very good at Newcastle, hence the 30M move to Spurs, but couldn't make that step up.
Everton is probably Sigurdsson's level.