Southwell's team return to Adams Park following a stirring victory over Premiership champions Harlequins at the Twickenham Stoop last time out, while Gloucester triumphed impressively on Northampton soil. Fourth-placed Wasps are just one spot and two points above Gloucester, which underlines Southwell's view on an encounter between two in-form teams.
"The Gloucester game this weekend is massive for both sides," Southwell said. "We are right next to each other in the table, and both of us are coming into the weekend on the back of good away wins."
He added: "We were obviously delighted to come back to training this week having got that win away at The Stoop, as we haven't won there in quite some time, but the mind has been focussed pretty quickly by the challenge we have on Sunday.
"Gloucester have threats all over the park, and you could see last weekend that their back-three are very dangerous, so we need to kick very well to them. We can't be loose there.
"We have improved a lot, I think, as a team since we last played Gloucester in September. It was early in the season when we weren't doing enough to win the tight games, and I think we have moved forward well from that."
Southwell leads a team showing two changes from the Quins game, with Italy centre Andrea Masi returning to midfield and fly-half Stephen Jones replacing Nicky Robinson. Gloucester, meanwhile, are unchanged.
Former England wing Mark Cueto believes Sale Sharks have started to "hit the ground running" in their quest for Premiership survival. Sale, five points adrift at the basement with eight games left, face another huge encounter when they tackle 10th-placed London Welsh in Oxford.
Victory over the Premiership newcomers would strengthen Sale's hopes of remaining in top-flight English rugby, with the Kassam Stadium clash coming just nine days after they beat Exeter.
Sale are without injured scrum-half Dwayne Peel, so Will Cliff deputises, and Scotland international Richie Gray is back at lock, while London Welsh field Gavin Henson in the number 10 shirt as he continues his recovery from injury.