Morgan is such a curious case. He is unbelievably talented and one of my favourite cricketers to watch when he is on. He was given a brief run in the Test side and failed to live up to expectations (16 matches at an average of 30). He had a good county season last year averaging 45, but has been incredibly underwhelming so far this year - 61 runs in 4 matches at an average of 10.17.
For me, all roads lead back to one man: Johnny Bairstow. Bairstow has played 7 county championship matches so far this year with an average of 100.66. He has scored 5 centuries (including another over the weekend), 3 fifties and a high score of 219*. I am not sure what more he can do to force his way into the team. He has been batting 5 for Yorkshire this year, so if he did replace Bell, I'd imagine England would look to move Root up the order.
If not Bairstow, other candidates include Alex Hales, James Taylor, James Vince and Daniel Bell-Drummond. Throwing youngsters like Vince and Bell-Drummond into the fire of an Ashes series is less than ideal, and Taylor, despite a century at the weekend, has been woefully short of runs.
If Lyth doesn't pan out, the most likely candidate to replace him would be Alex Hales. I like that he is right-handed as I believe there is an advantage of having a R/L combination at the top of the order. He is aggressive and in good form for Notts, averaging 46.2 in 8 matches.
Other candidates to replace Lyth would be a mixture of old retreads (Carberry and Compton) and new unknowns (Lees). Lees is still young and raw, and hasn't replicated last seasons form. Carberry is 34 and averaging just 33.47 for Hampshire this year. That leaves Nick Compton who is in form (averaging 40 for Middlesex) although at age 32 and having already been discarded by England once, I can't see them turning back to him again.
In reality, Lyth, at a minimum, will get the rest of this series to prove himself and most likely the tour of Pakistan too.
One thing that is certain, England's top order has been disastrous in 2015. In 11 of 13 Test innings this year, England have bettered 74/3 on only two occasions. In the four innings against Australia so far this series, the first 3 wickets have fallen for 43, 73, 29 and 42. That is not a recipe for success, especially against a potent attack like Australia.