The opening batsman had been an ever present in the club’s Southern Premier League 1st XI prior to the weekend.
Walker put on 56 for the first wicket with Andrew Galliers (19) and a further 51 for the second with teenager Charlie Whitfield (25) as Havant reach 107-2.
Bournemouth chipped away with some wickets to keep the score under control on a pitch that was difficult to start on as a batter.
Walker was eventually fifth out for a brilliant 90, having hit eight fours and a six in a 101-ball innings, and skipper Alex Whitfield and Graham Burns, who both finished unbeaten on 17, helped the hosts total 205-7 off their 40 overs.
Bournemouth’s reply started steadily with the openers Ollie Shrubsole (62) and Martin Miller (20) advancing the score to 52-0.
Warren Turner made the breakthrough, trapping Miller leg before. But that brought in Freddie Oldfield (44) who helped the visitors to 120-1 at the drinks/sanitiser break after 24 overs.
That was the prelude for Walker to be introduced into the attack.
He had only bowled one ball for Bashley in the truncated 2020 league season and turned his arm over briefly in two games for the same club in 2019. In 2018 his only overs had been for Dorset in a friendly against the Somerset Academy.
But his short spell changed the course of the game and set up Havant’s victory.
First he had Shrubsole leg before for 62 with a slower ball - his first wicket since taking 1-24 for Havant 1sts against Alton 1sts in May 2017 - then cleaned up Tom West (3) in his next over.
In his third over Walker removed Oldfied and Bournemouth were suddenly 137-4 after 30 overs.
Graham Burns chipped in with two quick wickets and produced a fantastic direct hit run out to reduce Bournemouth to 160-7.
Steve Matthews and Ali Gardener each picked up a wicket in their second spells before the returning Walker finished things off to end with 4-13 off 4.1 overs.
Bournemouth lost their last nine wickets for 62 runs in 15 overs to be bowled out for 186 and leave Havant 19-run winners.
It was the team’s fifth win in six completed Division 1 games and kept the pressure on top two Parley and Sway.
Sway remained second after a 69-run success over Burridge 2nds.
Captain Tim Noble top scored with 63 at the top of the order as Sway posted 223-5 after he had won the toss and elected to bat.
There was also a 60-run unbroken stand for the sixth wicket between Will Crossley (46 not out) and Oscar Marshall (22 not out).
Brenden Fourie was Burridge’s most successful bowler, but his two wickets cost him 53 runs in just six overs.
In reply, Burridge were up against it early on when openers Srujith Wickramasinghe (14) and Jack Slaughter (6) departed quickly to Jon Waller (2-11 off eight overs).
James Highes (33) and Francis Moore (24) put on 52 for the third wicket, but Burridge subsided from 78-2 to 120-8.
No 10 Stuart Downs (22) lifted the total to 154-9, with extras (26) the second highest score.
James Mitchell was the only Portsmouth 2nds batsman to impress as his side suffered a 63-run defeat to Winton.
Replying to the Dorset club’s 187-9, Mitchell - who opened the innings - hit 60. But only two other players - Ishafaque Ahmad (18) and Matt Shaw (17) - reached double figures as Portsmouth slipped to 124 all out (Charlie May 4-18).
Winton’s innings had also been dominated by one batsman - in their case No 3 Matt Clark who hit an unbeaten 88. The next highest score was 21.
Paul Hungerford (3-42), Billy Eales (2-34) and Joe Eales (2-36) were Portsmouth’s most successful bowlers as their team suffered a fourth defeat in six completed matches.