Centre-back Christensen’s dismissal just before the interval led to Fikayo Tomori coming on for the second half.
Tomori was given a place on the bench ahead of Antonio Rudiger, who was left out of the matchday squad altogether.
That is bound to increase doubts about Rudiger’s future at the Bridge, particularly in light of the recent arrival of Thiago Silva, who is building up his fitness following his move to Chelsea.
Lampard said: “Now we have five centre-halves at the club. I can’t pick a squad that has three of them on the bench.
“So that’s the way it is. I have to pick the squad by training, situations and opponents and that was the selection today.
“He did really well, Tomori, I have to say.
“He hasn’t played for a while and is striving for fitness. I was delighted with him.
“I hope he feels the trust. I’ve got confidence in him.”