The most intriguing dilemma is in midfield, something of a weak area for England for years, where Lancaster has to find the right blend of power and panache, experience and potential.
Certain of his place in the squad and the starting team is Jonathan Joseph, whose electric performances over the last year have softened the blow of losing the banned Manu Tuilagi.
Also in is Brad Barritt, the defensive enforcer who Lancaster holds in very high esteem.
Until this month it was assumed that Luther Burrell, Joseph’s regular partner in the Six Nations, was another certainty but it seems he might be still in the TBC department after a late run by Henry Slade and Sam Burgess, both of whom made impressive test debuts in the home warm-up win over France earlier this month.
Slade, who also covers the flyhalf position, is a classy performer with an eye for a gap and is reportedly pencilled in for a slot, whereas rugby league convert Burgess is still grasping the rudiments of the game but brings with him huge physicality and proven big-match temperament.
That was something sorely missing for much of England’s dire display in Paris last week but an impressive performance off the bench by Nick Easter may have been enough to earn the 37-old-old a berth.
Ben Morgan made his first appearance in eight months against France at Twickenham following a broken leg and though he was a key part of the set-up last year, his lack of match fitness might open the door for Easter to back up Billy Vunipola.
Easter’s case is helped by the fact he can play at blindside flanker and even lock.
Versatility is not always the advantage it seems, however, as flyhalf/fulback Danny Cipriani seems likely to discover.
Despite showing plenty of invention off the bench in Paris it would appear there is no room at the inn for the reformed bad boy of English rugby, with George Ford and Owen Farrell filling two flyhalf spots and Mike Browne and Alex Goode probably in as specialist fullbacks.
There is a tight call in the second row between the more athletic George Kruis and oak-hewn Dave Attwood while Jamie George looks set to claim the third hooker berth ahead of Luke Cowan-Dickie.
Lancaster is due to announce his squad at 12.45 GMT on Thursday and he is likely to play his first-choice team in the final warm-up match against Ireland on Sept. 5.
(Editing by: Amlan Chakraborty)