Sancho arrived for big money from Borussia Dortmund in the summer, but has failed to hit the ground running at the Old Trafford club.
He struggled to make an impact in the Carabao Cup against West Ham United in midweek, a game United lost 1-0 at home.
"I'm worried because the big man Rashford's on his way back," Agbonlahor told talkSPORT.
"If you've not started in the Premier League [Sancho was on the bench against West Ham in the Premier League last weekend] and Rashford's coming back, then he's going to be even further down the pecking order.
"Rashford will come straight back into the team with Greenwood on the right and Ronaldo in the middle.
"He's got to start putting in performances before Rashford becomes available."