Clark made a change in only the 23rd minute, substituting left-back Blackett after Blues conceded two quick goals.
"I sat down with him on Monday and told him the reasons," Clark revealed.
"I don't think it was the right time to speak to him on the day once I had made that decision.
"Tyler handled it really well. He handled it like a senior professional.
"We sat down and had a chat on Monday, I explained my reasons, I explained what I wanted from him. He understood and he is in contention for Reading. It's as simple as that.
"I have got to make these decisions sometimes.
"We did concede more than the two after Tyler came off but that was more down to bad luck because if my captain had still be on the pitch it wouldn't have gone to four, I'm convinced about that.
"But, you know, the early part of the game there were too many things happening that were causing me concern defensively."
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