Mjalli will head a committee investigating the Friday attack, The Jordan Times reported.
At least four people demonstrating against the government were wounded when government supporters attacked them. Mjalli promised to "expose the assailants" and bring them to justice.
In a speech to Jordan's political leaders Sunday, the king said, "Reform is our unwavering will."
He has told the Cabinet of new Prime Minister Marouf Bakhit to begin a process for "the emergence of political parties as well as increasing public participation in decision-making," especially in elections.
Opposition leaders tell al-Jazeera Abdullah seems to be heeding the lessons of Tunisia and Egypt.
"He took us by surprise. He was intent on listening to our criticism and demands. He also sounded enthusiastic about change," said one who met with him recently.
On top of the issues of poverty and repression that are fueling revolts across the Arab world, the 1994 peace treaty with Israel is highly unpopular in Jordan.
Mjalli has angered Israel by urging the release of a Jordanian soldier who killed Israeli schoolgirls in 1997.