The Japan forward matched his tally for last season thanks to a first-half double in the fourth-round tie at Parkhead.

Aaron Mooy also scored twice, with David Turnbull on target as well.

Even so, Morton had reason to feel aggrieved by the way Celtic went ahead from the penalty spot following a particularly harsh VAR decision against former Hoops defender Efe Ambrose.

Morton could have broken the deadlock when Lewis Strapp was left one-on-one with Joe Hart only to shoot straight at the former England goalkeeper.

At the other end, Celtic's Liel Abada had a shot blocked and play continued, with most spectators in the crowd thinking no more about the effort.

But VAR Craig Napier eventually spotted that Abada's shot had come off the foot of Jack Baird then bounced up away from goal and brushed Ambrose's arm, which was slightly away from his body.

That was enough for Napier to decide on-field referee Kevin Clancy had made a "clear and obvious" error.

Having looked at the pitchside monitor, Clancy awarded Celtic a penalty

Mooy sent Brian Schwake the wrong way from the spot in the 18th minute. Three minutes later Celtic were 2-0 ahead when Ambrose took a heavy touch on the edge of his box and Abada played in Furuhashi.

Celtic went further ahead in the 42nd minute when Furuhashi laid the ball back for Turnbull, who let fly with a first-time shot from 22 yards.

Furuhashi made it 4-0 in stoppage time after Grant Gillespie had taken a fresh-air swipe at Abada's cutback.

Morton, defending well, were on course to deny Celtic a second-half goal until six minutes from time when Mooy finished from close range after good build-up play by substitute Sead Haksabanovic.

Scottish Cup-holders Rangers, the arch Glasgow rivals of Celtic, are in fourth-round action later Saturday away to St Johnstone.