I don't think even Roman would have sacked him. Two cup finals lost on penalties to Liverpool + a CL quarter final lost narrowly to Madrid in extra time. The league performance was disappointing but I think once they fell out of the title race the focus went, they're clearly still the 3rd best team in the league IMO, although I expect us to change that next season with signings + them losing Rudiger etc.
Falling out of the title-race and any subsequent loss of focus would very much be the core issue, I'd suggest..