03/07/2015 09:01 EST | Updated 05/07/2015 05:59 EDT

Michigan coach Jim Harbaugh joins staff of Oakland A's for day at spring training in Arizona

MESA, Ariz. - Jim Harbaugh joined the Oakland Athletics as a guest coach at spring training, but only for a day.

The former coach of the San Francisco 49ers and new coach at Michigan was in full uniform Saturday, including high socks, for the A's. He coached first base for several innings against the Los Angeles Angels at HoHoKam Stadium.

Harbaugh wore No. 4, the number he used while playing in the NFL. He grew up in Palo Alto, California, and played on the same American Legion team as A's manager Bob Melvin, although it was a few years after Melvin.

"Let's play two, as Ernie Banks would say," Harbaugh said outside the A's dugout during pregame warmups.

They resumed their relationship after Melvin became A's manager in 2011. Harbaugh invited Melvin onto the 49ers sideline for several NFL games.