Towering midfielder Diop picked up his fifth domestic booking against West Brom after a first-half foul on Robert Koren.
It takes Diop to the bookings limit, earning him an automatic one-match suspension.
Had the forthcoming Newcastle fixture taken place as scheduled on the Saturday at Fratton Park, he would have been free to feature, regardless of his bookings tally.
A ban kicks in seven days after the last booking and subsequently, with the match moved for Sky to the Sunday, that rules him out.
Diop had also been booked in the Uefa Cup against Braga, although European competition yellow cards do not count towards the domestic scene.
Meanwhile, Johnson has now been booked four times this season after his yellow card at The Hawthorns.
It means the in-form right-back is just one more away from receiving a one-match suspension as well.
Other than those two, Pompey are relatively free from suspension worry heading into the busy Christmas period.
Lassana Diarra is next in line with three bookings, although he is not expected to return from an ankle injury until the turn of the year.
Sylvain Distin, Armand Traore, Younes Kaboul, Kanu and Glen Little have yet to receive a yellow card this season.
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The full article contains 253 words and appears in The News newspaper.