Van Graan admitted after the match that the defeat hurt, but he was confident that his side was closing the gap on the Dublin team.
Leinster won their last encounter 13-3, and although Munster dominated most of the match this time around, a late score by Jordan L’amour was the difference on the scoreboard.
“It hurts incredibly,” said Van Graan.
“It not only hurts me, it hurts everybody inside because the one thing you can never doubt with this club is the work they put in, the effort they put in, and unfortunately we feel the 50/50s didn’t go our way.
“That is rugby and that is life. We have to keep going, and the positive part is that it was a league game.
“Losing to Leinster is never good enough for Munster.
“We hope in the next one, whether that is in a final or the normal league game, or a European quarter-final - who knows what is next - we believe that hopefully we can put in a performance that, at that moment, goes our way.”
Leinster still looms large for every team in the Guinness PRO14, with the side racking up victories against Munster and Ulster after their stumble against Connacht a few weeks back, which makes them odds-on favourites to retain their title.
In Galway, Ospreys star Rhys Webb scored twice as the Welsh side stunned Connacht with a come-from-behind 26-20 victory on Sunday.
Connacht led by 12 points at one point against a 14-man Ospreys outfit, but the Welsh team found the resolve to respond early in the second half, and never looked back as they took a victory that will give them loads of confidence.
Over in Parma, Edinburgh fought to a satisfying 26-10 victory over Zebre without their coach Richard Cockerill and fullback Blair Kinghorn, who were forced to remain in Scotland as they awaited COVID-19 tests results.
Flyhalf Jaco van der Walt, who penned a new deal with the club last week, contributed 16 points in the victory.
Meanwhile, Cardiff Blues Interim Director of Rugby Dai Young celebrated his return to the club with a nail-biting 13-10 win over the Scarlets on Friday.
Matthew Morgan’s try, coupled with Jarrod Evans’ boot was enough to see the Welsh side home in their tightly contested derby, completing the double after their win in Cardiff a fortnight ago.
Guinness PRO14 results:
Scarlets 10-13 Cardiff Blues (Parc y Scarlets, Llanelli)
Zebre 10-26 Edinburgh (Stadio Sergio Lanfranchi, Parma)
Munster 10-13 Leinster (Thomond Park, Limerick)
Connacht 20-26 Ospreys (Sportsground, Galway)