United boss David Moyes secured the signature of the midfielder from his former club Everton on deadline day, but the club were criticised following the close of the window for only completing one major signing.
However, Round is confident that Fellaini, who is currently on international duty with Belgium, is the answer to the club's midfield problems.
"I am sure Marouane will settle in very quickly and be an integral part of United's success in the future, I really do," Steve told the Manchester Evening News.
"When you are building a squad to compete on all fronts you don't want every single player to be the same. You want options and differences.
"In a certain game it might be Tom Cleverley and [Michael] Carrick in midfield, it might be Fellaini and Carrick, or Fellaini and Anderson. There are different permutations It has given the manager options he didn't have before."
The Premier League champions also tried to sign Athletic Bilbao midfielder Ander Herrera, but a potential move fell through before the deadline.