The Trent Bridge showdown was rained off after just 18 overs, but there was still time for Kelly to produce the most incisive bowling spell of his seven-week stint at the County Ground.
Kelly, who will now be replaced in the Steelbacks squad by New Zealand all-rounder Jimmy Neesham, bowled superbly to take four for 29 as the Outlaws struggled to 140 for six before the rains came and called a halt to proceedings.
"Matt has been brilliant, and he has fitted in the dressing room from day one and been magnificent," said head coach Sadler.
"It's not always just about the numbers, the runs and the wickets, it is about the full package.
"Matt's attitude and professionalism, and the way he goes about his business, all credit to him.
"He has been an absolute diamond, so we wish him all the best.
"That was a fitting way for him to finish on Tuesday, getting that four-for, it was brilliant from him."
In all, Kelly played in five LV= Insurance County Championship matches, claiming eight wickets, and three Vitality Blast games, taking five wickets in all.
The Steelbacks are back in action on Wednesday night when they host Leicestershire Foxes at Wantage Road (start 6.30pm).
The point taken from the rained off clash in Nottingham means the Steelbacks currently sit fifth in the North Group table, with three points from three matches.
Leicestershire are in eighth place have won one and lost two of their three games so far.
The Foxes were badly beaten in their first two home games against Durham and Derbyshire, but then went to Headingley on Sunday and beat Yorkshire by 31 runs.