Saints boss Nigel Adkins sprang a surprise when he left out the club's top scorer for the past three seasons.
With his side 1-0 down at the home of the champions, Adkins eventually turned to the 30-year-old, who made an instant impact by equalising.
Lambert said: "I was disappointed not to start.
"I'd be lying if I said that I wasn't, but there's no sulking between the lads it's just a case of being ready for when you get your chance and that's what I've done.
"I was on the bench and I was just desperate to get my chance."
He added: "Everyone is fighting for the 11 places and the ones who are not in there are fighting to get onto the pitch."