Munster head coach Graham Rowntree admitted his side were a clear second best as they fell to their opening defeat of the season on Friday night against Ulster.

The defending BKT United Rugby Champions gave up a 14-3 first half lead against their rivals to lose 21-14 at Kingspan Stadium, as Ulster made amends for their own blown lead in an Interpro last week.

While Munster didn't actually trail until the 68th minute when Nick Timoney crossed for Ulster's second try, the hosts had been dominant for the entire second half where they controlled the breakdown, lineout and scrum, having struggled at times in the first 40 minutes.

And following their first defeat in any competition since the Champions Cup last-16 in April (v the Sharks), a frustrated Rowntree admitted it was an unacceptable performance from his side.

"We were inaccurate, sloppy, ill-disciplined," Rowntree said.

"Our ruck wasn't fast enough and that is the heartbeat of the game for us. Really disappointed. Our set-piece malfunctioned."

Despite the poor performance, the province came within inches of potentially snatching a draw when Josh Wycherley was held up over the line in the final 30 seconds of play.

But Rowntree took little consolation from the way they launched a late attack.

"Yeah, we were chasing the game at the end there, which I expect from us. We are never going to give up but we have to be better than that.

"We’re not pointing the finger at anyone else. We will have a good look at ourselves this week. That’s not good enough.

"We scored two tries early on and had some good territory early on without being brilliant. We weren't that good and we said that at half-time. There was certainly no complacency at half-time because they had already started to come back into the game and then the third quarter it just got away from us.

"They [Ulster] were positively putting pressure on us. We were piggy-backing penalties and getting put onto our own goal line, losing lineouts, we got run off our scrum and couldn't consistently string any phases together in the middle of the field.

"A hugely, hugely disappointed dressing-room down there. That just was not us. I said that to them. It’s just not us. We will own up, we will go away and we will get better for that," he added.

While Ulster had grown into the game late in the first half, there was a noticeable shift in momentum early in the second half when Dan McFarland's side introduced Ireland internationals Iain Henderson and Rob Herring, as well as the very impressive debutant Scott Wilson, off the bench on 49 minutes.

Conversely, Rowntree held back Peter O'Mahony, Conor Murray, Tadhg Beirne for this week, while Dave Kilcoyne was a travelling reserve.

And the Munster coach said he doesn't regret taking a gamble with his team selection.

"Those guys have played a lot of gametime in that World Cup and they were coming back into a short week.

"I stand by that. I stand by our selection this week. And we have those guys potentially coming back in next week. We will deal with that on Monday.

"They [Ulster] were certainly powerful. But I would be looking more at what we didn't do in terms of our ruck and our indiscipline that allowed them into the game."

The international quartet should return next week, as Munster welcome the Stormers to Thomond Park in a repeat of last season's final.

And Rowntree says he's already warned his players that a repeat of their Belfast performance won't be accepted.

"They certainly felt that from me.

"There's an honest group there. Nah, that wasn’t good enough and that is the headline from me as the head coach and from the players; not good enough."