Three second half goals from Burnley was more than enough to condemn Southampton to their first loss of the season at Turf Moor on Saturday.
An Ashley Barnes brace and a Gudmundsson goal in a glorious 12 minutes spell for Sean Dyche's men, handed Burnley the perfect start to the new campaign.
Nigeria-born Michael Obafemi, Southampton's youngest ever goalscorer in the Premier League, played the last 25 minutes of the closely fought battle but his best efforts could not rescue the Saints from succumbing to their clinical hosts.
The youngster, who committed his international career to Ireland last year after making his Premier League debut, was thrown into the fray to replace ex-Liverpool striker Danny Wings with the Saints already trailing to Barnes' brace.
However, only a few minutes after coming on, the 19-year-old could just watched as Gudmundsson went up the other end to net Burnley's and the match's third and final goal.
The Clarets thereafter kept their lead to see out a pleasing victory.
