Young goalkeeper Mason McCready starred for Thistle, both in normal time and in the penalty shoot-out in which the Championship visitors took the bonus point that was not enough to deny Well top spot in Group G.
Both sides passed up good chances to score, with McCready making two fine stops in the first half to deny Zach Robinson and Dan Casey.
The second-half was largely dominated by the visitors, with Daniel MacKay and Brian Graham spurning opportunities to put Kris Doolan's side ahead.
There was a moment of controversy when Thistle were prevented from fielding young striker Ricco Diack as a late substitute, throwing on Logan Chalmers instead, as he had not been included in a new online system.
After 90 minutes, it ended goalless and the game went to a meaningless penalty shoot-out, with the fate of both sides already sealed.
Motherwell contrived to miss three of their four spot-kicks, with Thistle's Aaron Muirhead and Ben Stanway also having their efforts saved by Aston Oxborough.
However, successful penalties from Kyle Turner, Brian Graham and Luke McBeth gave Thistle a rather pointless point in the end.
"Anybody at Motherwell Football Club who thinks that performance is acceptable and will gain you points in what is such a difficult league is going to get a rude awakening," Kettlewell told BBC Scotland before his side were drawn at home to Premiership rivals Kilmarnock in the next round.
"Existing players and new players will learn that's not what's accepted here. It's definitely not accepted from myself."