They both scored against Chelsea last weekend, and Collymore says they'll have to keep banging them in for United to be successful this season.
"But their start – a thumping 4-0 win over Chelsea, along with the performances of Paul Pogba, Marcus Rashford, Anthony Martial and Harry Maguire in particular – did hint that they might just surprise one or two of us.
"I'm not going to get carried away by one result and say they are now nailed on for the top four. The game against Wolves will be a bellwether to all of that.
"Success or failure for United this season will depend on their big characters – Pogba, Maguire and De Gea. But also to a certain extent on Rashford and Martial.
"Now it's time to send the same message to the United frontmen. Manager Solskjaer is looking for players he can put his money on, players he knows he can rely on, and those two need to step up to the plate.
"They will be full of confidence after their goals against Chelsea last weekend and I want to see both of them going north of 15 goals.
"Last season in the league it was 10 apiece – and that's not going to help United get to the sort of areas in the table they really should be.
"Ideally, both should be looking at 20 league goals as a minimum in a 38-game season, which is the benchmark for all top strikers."