The Labour leader was speaking on his first trip to Scotland since taking over from Jeremy Corbyn.
Senior Labour MP Rachel Reeves pointed to 'pretty dire' poll ratings for Scottish Labour.
Marco Biagi plans to go up against former SNP deputy leader Angus Robertson for the key Holyrood seat.
Carlaw steps down less than a year before the next Holyrood election.
Polling expert Professor John Curtice said 'never before' had public support for the union 'looked so weak'.
The Highlands politician, who has served as a councillor, MP and MSP, will step down aged 65.