Ole Gunnar Solskjaer's side are fifth in the Premier League but have enjoyed a resurgence under their former forward, while Barca are on track for successive La Liga titles.
Forlan – who won the Premier League and FA Cup during his two years at Old Trafford – fancies United's chances against the Spanish giants.
"Well they have a different game against Barcelona," Forlan said.
"But they are a really good team and in football you never know what's going to happen.
"They have a really good game in front, a difficult one. But they have a great team with good players.
"United do really well at home, they could have a good possibility for elimination [of Barcelona]."
After consecutive losses in the league and FA Cup, United return to action at home to Watford on Saturday while Barca – 10 points clear atop the table – welcome Catalan rivals Espanyol to Camp Nou.