In the first game of a day-night doubleheader, Jeremy Guthrie ties a franchise record by going 16 consecutive starts without a loss for the Royals. The 34 year-old right-hander works 6.2 innings without giving up a run in Kansas City’s 9-0 victory over Cleveland at Kauffman Stadium.

Bryce Harper makes his much-anticipated major league debut for the Nationals, but it’s the Dodgers’ Matt Kemp who steals the show, leading off the 10th inning with a walk-off homer off Tom Gorzelanny to give Los Angeles a 4 – 3 win. The 19-year-old Harper, the youngest player in the majors, hits a double in the 7th and then a go-ahead sacrifice fly in the 9th, but Washington’s bullpen blows a 3 – 1 lead, when Henry Rodriguez throws three wild pitches and gives up two runs to send the game into extra innings. Harper will go on to win the National League Rookie of the Year Award.

Major League Baseball announces a number of changes to the rules that govern the All-Star Game that have been agreed with the Players’ union: the designated hitter will now be used in all games, not just those played in American League parks; a pitcher who started a game on the last Sunday before the All-Star break will not be eligible to play in the game and will be replaced on the roster, although he will still be recognized as an All-Star (this will become known as the Sunday Starter rule); rosters are expanded to 34 players, adding one position player; one of the position players will be designated as being able to re-enter the game in case of injury – catchers are already allowed to do so in those circumstances.

3B Luke Hughes of the Minnesota Twins homers in his first major league at-bat off the Tigers’ Max Scherzer in the 3rd inning of an 11 – 6 loss. The Australian is the fifth Twin to accomplish the feat, but the first since Andre David in 1984. The Tigers trail 6 – 1 in the 4th and then score 10 unanswered runs. The key play occurs when Denard Span drops a fly ball hit by Johnny Damon in a six-run 6th inning.

Phillies slugger Ryan Howard agrees to a $125 million, five-year contract extension that could keep the infielder with the team until 2017. The 30 year-old first baseman’s $25 million guaranteed average salary during the new deal is the second-highest amount in the major leagues, trailing behind only Alex Rodriguez’s ten-year contract with the Yankees that averages $27.5 million per season.

Changes recommended by baseball’s special committee for on-field matters, including the expansion of All-Star rosters to 34 players, including 13 pitchers, will be implemented for this season’s Mid-Summer classic scheduled to be played at Angel Stadium in Anaheim. Other new adopted rules include the use of a designated hitter every year, making a pitcher who starts on the final Sunday prior to the break ineligible to play, and allowing re-entry into a game if the last available position player at any position is injured.