Last season, Sigurdsson netted nine goals and provided 13 assists as Swansea retained their Premier League status on the penultimate weekend of the campaign, but the Icelandic playmaker has now been sold to Everton for a club-record fee.
However, Mawson - who has impressed at the Liberty Stadium since signing from Barnsley in January - has said that Sigurdsson deserved to earn his move to a higher-profile team who are competing in Europe.
The 23-year-old told Sky Sports News: "He's a great guy, great player. He was massive for us last year and you could argue that if he wasn't in the team, we would have been relegated. That obvious for everyone to see given the quality he possesses.
"He deserved his move, he deserves to play for a club who has real ambition this year and Everton are going to do good things. They are playing in the Europa League, he deserves that at least.
"He's a big asset for them, he was for us, but when we come up against him, we know the quality that he possesses and we are going to have to try to nullify that."
Sigurdsson scored 30 goals in 112 appearances in all competitions during his time in South Wales.