NEW YORK (AP) - Johan Santana's final start of the season might have already taken place.

Santana is going to have an MRI on his back Tuesday, putting in jeopardy his scheduled start against the Colorado Rockies two days later.

The Mets had said Monday, following much internal discussion, the left-hander would continue pitching with his status being evaluated after each start. But manager Terry Collins scratched the plan after New York's 3-1 loss to the Rockies later that night - at least until the results of the medical tests are read.

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``He came in stiffer today than he had been,' Collins said. ``Actually, they got him loosened up. He threw his pen. He felt fine. When he was done, he got a little tight.'

The initial plan was for Santana to work on a strict pitch count. Collins said it would be somewhere below 100.

``From our standpoint we think Johan is good for another start or two and we'll take another look at the situation around the first of September,' Mets general manager Sandy Alderson said earlier Monday.

Santana has had an up-and-down season in his return from shoulder surgery that kept him out more than a year. He threw the team's first no-hitter on June 1, but it's been a series of exceedingly difficult starts the rest of the way. He is 3-7 with an 8.27 ERA since throwing a career-high 134 pitches in the no-hitter.

The two-time AL Cy Young Award winner was tripped up July 6 when he sprained his right ankle facing the Chicago Cubs. Including that start, he 0-5 with a 15.63 ERA, allowing at least six runs in each of the five outings. He is 6-9 with a 4.85 ERA in 21 starts overall.

``We have come a long way, and we know that. And I'm very happy with everything that I've been able to accomplish,' Santana said after his bullpen session, giving no indication he had further trouble with his back.

Santana spent three weeks on the disabled list after trying to pitch with the injured ankle for two starts - a DL stint that was supposed to give his arm a rest, too. He also has had some recent back tightness but said it was normal pain for a pitcher.

Still, the 33-year-old says he feels fine.

``I felt that I want to compete, keep pitching,' Santana said.

To give extra rest to Santana along with Chris Young, who had similar surgery to Santana last year, and rookie Matt Harvey, the Mets have begun using a six-man rotation.

While the six-man rotation is implemented, R.A. Dickey will pitch on his regular schedule to give him a couple of more chances at a possible 20-win season.

The knuckleballer was coming off a rough start at Cincinnati in which he was asked to remove two bracelets and matched season highs by giving up three homers and 10 hits.

He had little trouble with the Rockies' injury-depleted lineup Monday. The knuckleballer retired the first eight batters before Alex White looped a single over the outstretched glove of a leaping second baseman Daniel Murphy. He gave up three hits and a run and walked two in seven innings.

But a Mets offense that has averaged less than three runs a game over a 14-agme stretch provided him with little support and he was lifted for a pinch-hitter in the seventh inning of a tie game.

``It's tough, because the first half of the season, we scored a lot of runs and the offense really picked up the pitchers,' Dickey said. ``It would be great to have a couple of games where we don't hit the ball where the pitchers pick the hitters up. It doesn't always work that way, but we've got to find a way to manufacture some runs and we're not doing that right now.'

Murphy had an RBI single in the first but the Mets failed to get a runner to second again until the eighth inning when they loaded the bases against Rex Brothers (7-2).

Tyler Colvin, who hit a tying homer off Dickey in the fifth, made a diving stop on pinch-hitter Jordany Valdespin's sharp grounder just inside the bag with the bases loaded and his toss to reliever Matt Belisle just beat a diving Valdespin to the bag. Valdepsin jumped up, waving his arms in disbelief, and Collins engaged first base umpire Lance Barksdale in a prolonged discussion.

``They've all been told about you don't see track runners diving at the finish line. There's a reason they don't do it. It slows you down,' Collins said. ``Any time you hit the ground, it slows you down. Jordany gave it his best effort.'

Rockies manager Jim Tracy was impressed with Belisle's effort to make the play.

``After he finished his pitch he was a little bit off balance, and had to regroup and get over there to cover,' Tracy said.

Rafael Betancourt, the Rockies' fifth pitcher, put two runners on before getting Mike Baxter to fly out to center finishing his 24th save.

Josh Edgin (1-1) took the loss, giving up an unearned run in the eighth.

One of the factors in keeping Santana in the rotation is that they have no reason to put him on the disabled list. It is possible that the Mets could call up pitchers from Triple-A Buffalo to join the rotation when major league clubs are allowed to expand their 25-man rosters.

Santana is in the fifth season of a $137.5 million, six-year contract. He is due to make $25.5 million next year and the Mets hold a $25 million option for 2014 with a $5.5 million buyout.

Santana's deal includes a full no-trade clause. The 2014 option would become a player option if he pitches 215 innings next season.

NOTES: Mets 16-year-old signee German Rosario, a SS from the Dominican Republic, took batting practice with the team. He signed a $1.75 million contract.