NEW YORK -- Right-handed reliever Brandon Morrow rejoined the Mariners on Monday, but he was on the "unavailable" list for the Interleague series opener against the Mets at Shea Stadium.

The back spasms that kept the fill-in closer out of the weekend three-game series against the Braves appear to be behind him. Morrow went through an extensive throwing session on flat ground in shallow left field prior to pregame batting practice on Monday afternoon.

"I threw some 'pitches' hard and everything felt good," said Morrow, who is filling in as the closer until right-hander J.J. Putz comes off the 15-day disabled list, probably after the four-day All-Star break next month.

Morrow, who leads the Seattle pitching staff with a 0.90 ERA in 24 appearances this season, said the back spasms apparently were related to the BP that Mariners pitchers had last Wednesday at Safeco Field, leading into the nine-game stretch of Interleague games played in National League ballparks -- Atlanta, New York and San Diego -- where the designated hitter rule is not used.

Morrow said he started to feel discomfort when he bent down to pick up some baseballs after the BP session.

"It started to tighten up, and I tried to play catch in the outfield, but it got worse," he said. "I went in, and they put ice on my back for about 20 minutes, and that's when it really locked up. It was awful. Wednesday was the worst."

When it became apparent on Thursday that he would not be available for a few days, Morrow remained in Seattle for treatment, while the remainder of the club traveled to Atlanta for the Interleague series.

The Mariners did not need a closer in the series opener as they beat the Braves, 10-2. However, Morrow would have been used on Saturday night, when Seattle led, 4-1, in the eighth inning, but it ended up losing, 5-4, as the Braves scored twice in the eighth and ninth innings.

"I wish I could have been there to help out," said Morrow, who said he watched the games on television.

If Morrow feels good on Tuesday morning, he would be available to pitch in the middle game of the three-game series against the Mets.

Asked if he might volunteer for duty in the series opener, Morrow said he wouldn't raise his hand, even if he wanted to.

"They were pretty firm about it," he said of being off limits for the game.

Mariners interim manager Jim Riggleman said he and pitching coach Mel Stottlemyre want to make sure Morrow is 100 percent healthy before putting him back into a game situation.

"He had a pretty intensive throw session," Riggleman said. "We didn't want him to take it easy and not know if he could pitch. He stretched it out pretty good, and we're going to shut him down tonight and maybe have him available tomorrow.

"We won't use him tonight, regardless."