Ole Gunnar Solskjaer insists Manchester United were right to sell Romelu Lukaku last summer.
The Belgium international was sold to Inter was £74m before the start of the season, with United opting against signing a replacement.
"We always believed that Anthony and Marcus were going to score goals. Mason has filled a big void, he has chipped in with goals," Solskjaer said.
"I'm not worried about goals from the centre forwards. Mason has eight now, Marcus and Anthony have quite a few.
"Romelu is a good striker and he will always score goals but it was time for him to move on. We didn't find the right one [replacement] outside of our club - we had Mason.
"If we can keep these lot fit and keep on improving, one or two additions could create competition for places," he added on United's plans for the January transfer window.
"That's what we have to deal with at this club. We haven't much (competition) because we've had injuries but now I have to think about leaving players out. It gives me more options.
"I am optimistic that we can keep the players fit and if the right ones (transfer targets) come about I do have the resources and the backing from the board."
Solskjaer's side travel to the Emirates Stadium to face Arsenal on New Year's Day, before facing Wolves in the FA Cup this weekend.
