The Saints academy graduate played a key role in Sunday's win as he provided the assist for Charlie Austin's early strike, which proved to be the winner as the hosts recorded a 1-0 success.
Sims put in a man-of-the-match display for Claude Puel's men, although he admitted he was taking aim himself rather than trying to set up Austin for his assist.
He told the BBC: "The chance fell to me really. It might have been a shot, but I'll take the assist."
"I felt relaxed really and I settled into the game really quickly with the assist obviously helping," he added.
"I wanted to impress on my debut, but most importantly, I wanted to keep it simple to begin with and work my way into the game. But, I think everyone out there today was brilliant which really helped me.
"I know what my game is and I just played how I have been playing for the under-23s and that's why I've been given my chance."