Merrett, who was due to play his first pre-season game on Saturday for AFLQ team the Aspley Hornets, said Henderson would be perfect to help take the pressure off undersized centre half back Joel Patfull.
Merrett and Patfull were the backbone of a Lions defensive unit which surprised many last season, but the club is on the hunt to move Patfull to the third 'tall'.
"Lachie has stepped into that fullback role (in the pre-season) and done well and hopefully he can cement a position,'' Merrett said.
"I think he has the size, he's a good mark and will back himself. He needs that confidence in himself and I think he can do the job.''
Merrett said he loved playing alongside Patfull, who regularly punched above his weight last season, but the 195cm, 91kg Henderson offered another big body.
The 24-year-old Lions vice-captain was scheduled to take to Griffin Park at Coorparoo on Saturday afternoon against Morningside and said he was excited about playing competitive football again.
"I'll have the GPS on and get my kilometres up,'' he said.
"I'll make sure I play down back and be accountable and get back to basics and get that fatigue in the body again.
"I'll definitely be sucking in the seagulls. Just working under fatigue, that's what I want to get out of it because that's what I'll be like on March 28 against West Coast.''
Merrett said he expected to play half a game, possibly more, depending on the advice from the club's conditioning staff.
The reliable fullback's pre-season was disrupted because a lingering knee injury courtesy of tightness in his thigh.
Merrett said there would be no problem entering round one.
"I've got confidence in the training I've done that I will be able to compete well against West Coast," he said.
"I only played one pre-season game last year as well."
