For all of their early possession, Chelsea did little to test Keylor Navas in the Forest goal, and their passive attacking play would be punished with just 13 minutes on the clock.
Lampard elected to promote Edouard Mendy to the starting lineup in place of Kepa Arrizabalaga, but the decision soon backfired, as the Senegal international came for Renan Lodi's cross, missed, and Awoniyi gratefully headed home into an unprotected goal.
Rustiness perhaps came into play for a man who was making his first start in any competition for five months, but Chelsea briefly responded well to that early setback.
The Tricky Trees survived two efforts from Raheem Sterling and Lewis Hall - two other players recalled to the first XI - with 18 minutes gone, as the former was denied by a terrific intervention from Joe Worrall before Hall failed to steer his strike beyond a sea of red shirts.
The Blues' age-old deficiencies in the final third were evidently harming them once again, but Hall continued his bright start with a pinpoint cross to the head of Joao Felix in the 33rd minute, but Navas denied the Atletico Madrid loanee with a terrific full-length save.
That chance would prove to be Chelsea's best opportunity of an otherwise flat first-half display, and the Blues - who lost N'Golo Kante to a season-ending groin issue before the game - were dealt another injury blow towards the end of the opening 45, as Mateo Kovacic was forced off to be replaced by Ruben Loftus-Cheek.
Only a couple of moments after Moussa Niakhate missed a golden chance to make it 2-0 to Nottingham Forest early in the second period, Lampard's half-time team talk paid dividends, as Trevoh Chalobah got to the byline and cut back for an unmarked Sterling to fire home the equaliser in the 51st minute, albeit with the help of a significant deflection off of Ryan Yates.
Sterling's leveller was his first top-flight goal since New Year's Day - also against Forest at the City Ground - and as Chelsea began to take the game by the scruff of the neck, the 28-year-old soon doubled his tally for the afternoon in the 58th minute.
A quick Blues break saw Loftus-Cheek slip through Sterling, who left a sliding Felipe on his backside before curling a sublime effort into the far corner from the left-hand side of the box.
However, delight turned to dismay for the home crowd just four minutes after Sterling's second, as Chelsea failed to deal with a long throw into the box, and the ball found its way to Orel Mangala to cross for Awoniyi's second header of the afternoon.
Following that 11-minute mad-gap goal rush, the game slowed to a crawl as Forest's attempts to shut up shop proved effective,