The Scottish striker has not featured for the Blades since playing against Burton Albion in September.
He has struggled for a regular starting role ahead of Billy Sharp and Conor Sammon under manager Nigel Adkins.
The 23-year-old – who will be reunited with former Blades Ben Davies and Michael Doyle at Pompey – will stay at Fratton Park until January. McNulty has made eight appearance for United this season, netting his only goal in the 2-0 win over Blackpool in August
Today, the Blades welcome non-league Worcester City to Bramall Lane, keen to avoid an FA Cup giantkilling.
Back-to-back defeats to Millwall and Crewe have dented their promotion challenge, and Adkins is eager to get back to winning ways.
“We are expected to win every game, Saturday’s no different,” he said. “It is the FA Cup, everyone knows the beauty of it and as a club we are excited to be in the competition and determined to embrace it.
“This competition is always packed with drama, fairytales and upsets. Look at what this club achieved two seasons ago, when it reached the semi-finals.
“The history is there for everyone to go and see and as the next game we are looking forward to it and determined to win the fixture.
“Worcester have a few experienced heads and they obviously have something about them as a team to have come this far. They have a good cup pedigree; they beat Coventry City last season and pushed Scunthorpe United all the way to penalties, so they will certainly get our respect.”