United were dealt a blow earlier in the week as last season's top goalscorer Roofe departed for Belgian side Anderlecht with just twelve months left to run on his deal in West Yorkshire.
The 26-year-old bagged 15 goals last term under Bielsa, despite struggling with a number of injuries across the campaign.
The Whites moved quickly to ensure a replacement was found in the Argentine's squad as they beat strong competition to land Nketiah's signature for the season.
Bielsa, though, says there won't be any comparisons between the two as the 20-year-old makes his first steps into a full season of senior football in LS11.
"If (Kemar) Roofe was here we would not sign a third striker," Bielsa admitted.
"We cannot forget Roofe only played half of last season and in this half he scored 15 goals. If he continued making the same goals in the matches he missed he would be one of the most important scorers in the league.
"I don’t want to give that responsibility to another player. Of course, as a striker, they have to be linked with their number of goals."
Nketiah, though, says he is looking forward to the challenge: “It’s a different change.
“I’ve been at Arsenal for a while now in and around the first team, on the fringes and played a bit so I feel like it was important this season for me to go somewhere where I feel like I can get consistent game time and take the next step in my development. I feel like I can do that here.
“Also, there is a big pressure here to try and get promotion and I feel like it will be good for me to learn how to play under that and deal with that."