Diack was Ulster's stand-out performer at Ravenhill on Tuesday night but his efforts couldn't prevent the home side from slipping to a fourth consecutive Magners League defeat.
Diack was a constant thorn in the Ospreys' side in what was just the fourth time the South African had played a full 80 minutes in the Magners League all season but the visitors still ran out 38-27 winners to move up to second in the table.
To make matters worse, a late Tommy Bowe score robbed Ulster of a losing bonus-point and ensured they remain locked in a tight battle with Connacht for Ireland's third and final Heineken Cup spot next season.
"It was a huge pressure game for me," Diack told the Belfast Telegraph. "I got the opportunity and I played well, but I'm bitterly disappointed because this game meant a lot to us.
"We put a lot of effort in and we are very disappointed with the result and also not getting that bonus point."
Ulster's current form is starting to worry the Ravenhill faithful, who have seen their Magners League Play-Off hopes evaporate over the last few weeks.
Instead of pressing for a top-four finish, Diack and co are now being forced to look over their shoulders towards an in-form Connacht team that finally seem to have made a habit of picking up points, particularly at the Sportsground.
Ulster now travel to Play-Off chasing Glasgow this Friday, with just four points separating them from their Galway-based rivals and with Connacht still having a game in hand.
"It's just simple discipline," added Diack when asked why results aren't going Ulster's way.
"We have got to improve on that and we have been saying it every week.
"It's just a matter of when we start learning from these mistakes, but you can't fault the guys for their effort.
"It's a huge disappointment to get nothing from the Ospreys game. At half-time we were close and with 20 minutes left we were very close and in the lead.
"With two minutes to go we were still in with a bonus point shot and then you get that last runaway try from Tommy Bowe which made it even more gut wrenching.
"We need to put in a huge performance against Glasgow - it's going to be a big one."
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