The 61-year-old boss has undergone heart surgery and faces a period of recuperation lasting at least a few weeks, while the Robins are also without Kyle Storer, who remains suspended.
Daniel O'Shaughnessy is an injury doubt, having suffered a foot injury in Tuesday's 3-0 defeat at Exeter. Jack Munns could replace him.
Coach Russell Milton, who has been placed in temporary charge while Johnson recovers, otherwise has a near fully-fit squad from which to choose.
Max Clark misses out for Cambridge after being sent off in Tuesday's 1-0 home defeat by Hartlepool.
Should manager Shaun Derry respond to that result by making further changes, Medy Elito will be expected to start.
He scored a hat-trick as a substitute during last weekend's 3-0 win at Carlisle, but was again named on the bench on Tuesday.
Derry is also expected to be without Barry Corr, because of a groin injury, and long-term absentees Uche Ikpeazu (hamstring), Josh Coulson (ankle) and Adam McGurk (thigh).