The Red Devils have seen Rasmus Hojlund and Joshua Zirkzee struggle with their goal tallies this season.
Hojlund has scored just twice in 18 Premier League matches, while Zirkzee has netted three in 23.
“Absolutely, we take the responsibility,” Martinez told reporters after a 1-0 win over Fulham.
“I think we played many long balls to them, and for sure it’s tough for them, especially because they fight against two centre-backs. The centre-backs here in the Premier League are really difficult.
“But they (Hojlund and Zirkzee) have to do better as well. They are really good players, but I agree with you. I think we have to continue (to do) more, and put them in better spaces on the pitch.
“We have to find them in the right places, dangerous places, to make them score.”