It was announced on Friday that Locke had signed a three-year contract to stay on as boss after initially taking charge on an interim basis in February.
The former Kilmarnock player was brought back down to earth 24 hours later, when Motherwell's Stephen Pearson celebrated signing a new two-year deal of his own with a goal and Lee Erwin won it 13 minutes from time after David Syme had initially put the home side in front.
Victory for Motherwell moved them seven points clear of bottom side St Mirren, who look destined for relegation to the Championship.
At the top, Aberdeen's hopes of overhauling leaders Celtic look even more slim after they were held to a goalless draw by Partick Thistle.
Derek McInnes' side are seven points adrift of the champions, who have a game in hand as they close in on yet another title.
Third-placed Inverness Caledonian Thistle are now without a win in five top-flight matches following their 1-1 draw at home to Dundee.
Paul McGowan put Dundee in front eight minutes into the second half, but Graeme Shinnie struck two minutes later to deny the home side.
Dundee United's miserable form continued as Ross County sent them to a ninth straight winless game by triumphing 2-1 at Tannadice, courtesy of goals from Jackson Irvine and Raffaele De Vita.
St Johnstone are a point behind United in fifth after drawing 1-1 at 10-man Hamilton, who had player-manager Martin Canning sent off.
