Sessions made the decision on recusal after he took office. He consulted with the DOJ legal ethics committee that recommended he recuse himself.
Sessions was named in November, November 18. I don't recall any "Russia investigation" being on the radar at that point in time.
Sessions was asked about Russia during his confirmation (Trump faults Sessions for his answer to Franken), but I don't believe any suggestion about the campaign being under investigation had surfaced yet. At most, Flynn was viewed as "Russia dodgey," but Flynn didn't resign until Feb 13, weeks after Sessions' confirmation hearings.
In Trump's statement, he blames Sessions for not apprising Trump that Sessions was bound to recuse - that Sessions' omission was "unfair -- and that's a mild word."
You mean same the DOJ lawyers and employees who former Speaker New Gingrich said contributed 97% of their political dollars to Hillary's campaign?
The same DOJ "ethics committee" who said not a word about Holder's Contempt of Congress, or Lynch's clear violation of the law by protecting Hillary?
Come on.
If Sessions blew over that easily, no wonder he isn't up to the job.
And considering the timing of when it all happened, I have no doubt everybody on that legal ethics committee that made the recommendation was a loyal servant during the Obama administration. Again, he comes across as naive. That he's got a solid reputation of conservative principles doesn't mean he's competent to wage the battles he's needing to fight at this level.