The deal includes an option for a permanent transfer next winter, which, a person close to the situation said, would cost about $5 million.
It also comes as the club weighs trade offers for striker Ola Kamara, MLS’s co-leading scorer last season who is in the final year of his contract.
“Bolstering our attack has been a priority for us in the offseason, and we’ve been closely monitoring the possibility of bringing Michael to the club,” Dave Kasper, United’s president of soccer operations, said in a statement. “He is a dangerous [striker] who we believe will be a great addition to our attacking unit.