The Brumbies haven't won two games in a row in Canberra since the 2016 campaign and haven't won consecutive matches anywhere this season.
It's a trend the Brumbies are desperate to change and they risk having their season slip away if they cannot topple the unpredictable Jaguares.
The Brumbies have a run of three home games in a row that can make or break their season, with matches against the Jaguares, Canterbury Crusaders and Melbourne Rebels.
Forwards coach Laurie Fisher said the players needed to be mentally tough to end a form rollercoaster ride.
"We have to be more clinical … it's a mix of forced and unforced errors at the moment," Fisher said.
"We probably don't consistently work hard enough across the board to have the threats across the field the way we want.
"We just let ourselves down by mixing some very good with some not very good. Everyone will make mistakes, but we need to get the unforced errors out of the game, make less of them at important times.
"I think it's a matter of mentally pushing yourself to be in the right place at the right time, every time. I don't believe it's a fitness issue for us, it's more of a concentration thing.
"We want to be a better communicating side and organising ourselves on and off the ball, if we can activate ourselves with a bit more voice, it demands people get into better spots."
McKellar will finalise his team on Friday, with Speight likely to return to the wing after missing games against the Queensland Reds and Otago Highlanders.
The flying Fijian will give the Brumbies a boost after they scored just one try last weekend. Seven of their past eight tries have been scored by forwards.
McKellar is also expected to re-inject captain Sam Carter into the starting side and weigh up his back line combinations for the Jaguares, who shocked the Melbourne Rebels last week.
The Jaguares will also have inside information on the Brumbies given new coach Mario Ledesma was a regular at training in Canberra less than a year ago in his role as Wallabies forwards coach.
Ledesma oversaw Allan Alaalatoa's switch to tighthead prop, the rise of loosehead prop Scott Sio and the Wallabies introduction for Rory Arnold.
"We've seen how the Jaguares play this year … they like to scrummage. They're not big hitting, but they're heavy in the ground and generate massive power the longer a scrum goes," Fisher said.
"Lineout wise, I think Mario's taken what he was trying to deliver in Australia. [Ledesma] will have a good understanding of all the provincial sides and that might have put him in good stead last week.
"He understands how we prefer to play and we think we've got a good understanding of how they play. I don't think there are too many secrets, it's just a matter of who can produce on the day."
SUPER RUGBY ROUND 10
Sunday: ACT Brumbies v Argentina Jaguares at Canberra Stadium, 4.05pm. Tickets from Ticketek.
Chris Dutton is a sports reporter at The Canberra Times.
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