Man United landed three trophies during Jose Mourinho's first season at the helm, in addition to securing qualification for the 2017-18 Champions League after beating Ajax 2-0 in the final of the Europa League on Wednesday night.
The Red Devils are expected to kick off their summer spending with a move for Atletico Madrid attacker Griezmann, but Capello has said that Mourinho's side are still a long way off matching Chelsea.
"It was a poor game," Capello told the Daily Star. "There was not enough exciting football. United are strong and defend well, which is good. But they are not an exciting team.
"Griezmann is a very good player. But what is his best position? He is not a striker. United need more than him. They have other areas in which to strengthen. Chelsea are so strong and they need to catch them. It will be difficult."
Man United finished sixth in the Premier League last season, some 24 points behind the winners Chelsea.