El Alagui struck past Stuart Nelson following Harlee Dean's lay-off in the fifth minute of injury time to maintain Brentford's unbeaten start to the season.
After spending big over the summer on a number of new faces, including Walsall sharpshooter Will Grigg, the Bees were hotly-tipped to go up this season.
But Rosler believes improvements must be made if his side, who fell behind to Adebayo Akinfenwa's opener, are to set the pace.
"I'd be happy with the point, especially when you get itin injury time," he said.
"It was not lucky, because we hit the target nine times from 20 attempts, but overall I think I'm disappointed with our performance.
"We talked about the same thing last season, that we're not converting our good play into enough goals.
"We had enough chances and we are also talking about the opposition, who score a goal out of nothing.
"The players are getting more games under their belt, they're getting more experience and eventually we will score more goals. In the end, I take that point.
"It's always good to come back, it's always good to get a goal so late, we've dominated in attempts and chances, but everybody in our team knows we have to play better and be more dominant. We are not the finished article yet."
Akinfenwa slotted home from close range after 40 minutes following neat interplay between Myles Weston and Danny Kedwell before Bees goalkeeper David Button denied the same two players from doubling the Kent side's advantage.
For Brentford, Dean's header failed to trouble Nelson after he latched on to George Saville's free-kick in the opening minutes, while Chris Whelpdale's instinctive block denied Clayton Donaldson an injury-time equaliser shortly before El Alagui earned the Bees a point.