Chelsea defeated Atletico Madrid, Porto, Real Madrid and Manchester City on their way to lifting the Champions League last season.
The Blues are close to signing Lukaku, who left Chelsea in 2014, this summer.
Lukaku is expected to complete his medicals at Chelsea ahead of an imminent move.
Speaking on the development, Merson reckons more success could be around the corner for Chelsea this season as they prepare to unveil Lukaku as their new big-money signing.
“The defending champions, Chelsea, will definitely be in the running for the UEFA Champions League this season, but only if they manage to snap up Romelu Lukaku,” Merson told Sportskeeda.
“Lukaku is a fantastic striker and will guarantee you 20-25 goals a season. No one else can do that for Chelsea right now.
“Chelsea will be a completely different proposition [this season] if they manage to get someone like him, so we will have to wait and see how this one pans out.
“It is hard to win the UEFA Champions League once, let alone in successive seasons.
“Some of the strongest teams in history have failed to win the competition [Champions League] back-to-back, but Chelsea will be in the reckoning, in my opinion.”
