Not only were Grant and Sherman not cut out for civilian life, at this time if you were to compile a list of general officers who would probably lead the Union Army (even with the assumption the southern officers would defect), Grant and Sherman would not make the list.
I don’t know who would be on that list. The three I thought might be frontrunners - Halleck, Fremont and McClellan - all resigned from the army before 1858 and didn’t return to the service until the war started. No wonder Lee was the first choice of the War Dept to be commander in chief.