Firpo has been the only major new signing of the summer so far - not that coach Bielsa seems in any way unhappy about that - and is the only specialist left-back at a club which effectively did without one in their first season back in the Premier League.
Firpo and centre-back Diego Llorente both missed the final two matches of pre-season and only the latter is expected to feature at Old Trafford. Bielsa expects to be without the Spaniard for "one or two weeks".
Phillips has had a truncated pre-season but has needed less work to be ready for 2021-22 having been an ever-present in the England team which reached the final of this summer's European Championship.
Bielsa can be quite fastidious about the fitness demands he places on players to select them for first-team duty but says the midfielder is "under consideration" after working "in a serious manner and with total dedication” since he returned to Thorp Arch.
Jamie Shackleton is recovered from a pre-season head injury and an option to start if Bielsa decides to limit Phillips to the bench.
German international Koch often filled in as the holding midfielder in pre-season but may be required in his prefered position in the heart of defence in Llorente's absence.
Llorente and Koch both joined last summer but largely due to injuries the two right-sided centre-backs only started together twice, in early May when Koch was used in front of the Spaniard, helping his fitness ahead of his European Championship call-up.
In truth, whether Bielsa would sign a new contract for a fourth season has been little more than a technicality with his commitment to the club made clear again over the summer when his work extended to coaching the club's youngsters and overseeing upgrades to the training ground.
The club say he is yet to sign a new deal, but Bielsa declared the matter "already resolved." Asked if it would be another one-year contract, he replied that was "habitual".