Motherwell 1 Dundee Utd 2

Keith Watson answered a late call to send Dundee United into the fifth round of the William Hill Scottish Cup following an emphatic second-half comeback against Motherwell.

Watson was named on the bench but was promoted just before kick-off after Jaroslaw Fojut suffered an injury in the warm-up.

And he proved the hero when he headed home fellow full-back Conor Townsend's cross with eight minutes left to seal a 2-1 victory at Fir Park for last season's runners-up.

Henrik Ojamaa had fired Motherwell into an early lead and the hosts' performance belied their lowly league status in the first half, but even then they were conceding chances before being utterly dominated after the break.

Goalkeeper Dan Twardzik kept them ahead until midway through the second half when John Souttar headed home and there was little surprise when Watson swooped.

The defeat adds to Motherwell's financial struggles as they await concrete news of a takeover that will allow them to choose a successor to Stuart McCall.

Caretaker boss Kenny Black had them well-organised and motivated form the start, though, and there were barely six minutes on the clock when Motherwell struck after a lengthy spell of possession inside the United half.

Paul Lawson rolled the ball to Ojamaa, who had one thought on his mind as he took a touch 25 yards out and drilled a shot just inside the post.

Motherwell continued to pass the ball about well but United always looked dangerous when they broke forward quickly and they carved a chance moments after the opener when Chris Erskine played Gary Mackay-Steven through, but Stephen

McManus made a crucial sliding challenge. United goalkeeper Radoslaw Cierzniak held a Mark O'Brien header just beneath the bar before Iain Vigurs and Paul Paton were booked for a confrontation after the Motherwell midfielder accused his opponent of making the most of a challenge.

Josh Law headed straight at Cierzniak from six yards following Zaine Francis-Angol's cross before United again hit their hosts on the break but Stuart Armstrong shot inches wide after a one-two with Erskine put him one on one with Twardzik.

The game continued in a similar vein with Cierzniak saving from Law after another controlled passing move from the hosts before United again cut them open when Armstrong played in Ryan Dow, but Twardzik stopped the shot with his foot.

The action continued apace. Erskine had a shot deflected just wide before Keith Lasley and Conor Townsend were booked and there were no takers for a brilliant drilled cross from Vigurs, who escaped a second yellow for a trip on Mackay-Steven on the halfway line.

Motherwell suffered a blow seconds after the restart when Lawson pulled up injured after an impressive display.

United took control and Erskine had a couple of efforts from just inside the box with Twardzik producing an excellent diving save from the second. The visitors should have levelled when Mackay-Steven took Erskine's pass in his stride to move inside Francis-Angol but the winger shot too close to the goalkeeper and Twardzik palmed the ball away.

The equaliser came from a corner straight into the six-yard box which Souttar ran on to and met unchallenged to plant a header past Twardzik. The visitors grew even more dominant and Twardzik blocked from Callum Butcher after the substitute had got the better of Simon Ramsden.

Aidan Connolly fired just over before Twardzik made one-on-one saves from Dow and Armstrong, the latter when he dived at the midfielder's feet after he had been left completely exposed by Dow's through-ball.

Motherwell were camped in their own half and the winner came from a deep cross from Townsend. Watson got across the static Francis-Angol to head the ball down and into the net from 10 yards.

Watson made another vital contribution as he did enough to put off Sutton as the striker headed over an stoppage-time chance from eight yards.