People forget Doug is a business man, if he is advised Rushworth is the next big thing in goalkeeping and he feels there is value in 5.5m then he could have made a speculative offer. It's low-ish risk and potentially a quick turnaround for a big profit especially as we have Dovin back training within 6 months.
As it is a loan works well for all parties anyway, less pressure to rush Dovin back early now. Wilson can slip back to 2nd choice in the meantime and hopefully Collins will want to move